For those looking for a very large toy hauler, it would be hard to beat this Jayco Seismic 399, a new model for 2025. With 13 feet 6 inches of garage space and enough sleeping capacity for nine people (along with two bathrooms), this toy hauler certainly looks great on the stat sheet. But what’s even better is that the attention to details and a number of features you’ll discover in the Seismic 399 make it a great choice in real life.
At a Glance: Seismic 399
While this is truly an enormous toy hauler, that works for some people. (Specifically, people who have toys and love fifth wheels.) If this is what you’re looking for, I would say the 2025 Jayco Seismic 399 is a four out of five. There is so much attention to detail in here and so many things Jayco has done right that it’s hard to find fault with it in this size range. However, there is one thing that may lead to swearing, if you’re parked and have the slides out.
Check the Specs
- Sleeping capacity: 9
- Hitch weight: 3,370 pounds
- UVW: 14,950 pounds
- CCC: 5,540 pounds
- Exterior length: 45 feet 11 inches
- Exterior height: 12 feet 8 inches
- Exterior width: 7 feet 2 inches
- Fresh/Gray/Black: 106/87/87 gallons
- Propane: 60 pounds (in two tanks)
- Awnings: 2 x 11 feet, plus a full-width awning over the patio deck
Features We Love
- Dexter Tow Assist system
- JaySMART lighting system
- High-quality suspension and tires
- Rear service panel for the refrigerator
- Fueling station with standard fuel handle
- Rear patio deck with three season patio doors
- Kitchen design and appliances
- Premium interior feel
- Jayco moved the furnace vents off the floor for 2025
- Cargo area with additional sleeping capacity and separate bathroom
- Norco Exact Slide mechanism on main slide room
- Observation camera in the cargo bay
Benefits
On the Outside
- Dexter’s Tow Assist system uses technology from Bosch to provide anti-lock braking and sway mitigation on this trailer, a significant safety advance in towable RVs. While newer vehicles all incorporate anti-lock braking, this is a newer feature in towables. (And it can make for much safer towing.) Another feature is the ability to track mileage with this system which can help with routine maintenance.
- Jayco pioneered their JaySMART lighting. (The acronym stands for Safety Markers and Reverse Travel.) This system flashes upper and side marker lights when you activate the turn signal. It visually notifies other drivers to the fact that this big beast is about to make a turn or change lanes.
- The suspension on this RV Rota-Flex King Pin Box, Dexter Axles with ABS and sway mitigation, Dexter heavy-duty shackles with wet bolt fasteners and bronze bushings and 17.5-inch Uniroyal H-rated 16-ply tires. (Got all that?) The Dexter heavy-duty shackles with wet bolt fasteners and bronze bushings help resist the wear and tear of towing. The Uniroyal tires provide a safer ride, far fewer repairs and much less sway. Further, the cushioned shackles reduce vibration and road irregularities that transmit into the RV itself thereby improving longevity.
- The service panel behind the refrigerator enables you to access the back of the fridge for service or winterization without having to take the whole thing out.
- The fuel station in this allows you to fuel-up your toys in the field and also provides fuel for the built-in generator. There’s a proper fuel handle like at a gas station and a fuel pump.
- Out back the ramp that you would use to drive in your toys can also double as a patio deck complete with side railings. There is also a three-season glass patio door with windows that open. That way, you can leave the deck down the entire trip and still block off Mother Nature.
On the Inside
- With the design of the kitchen, it would be possible to create nice meals even if you brought eight of your closest friends.
- Many toy haulers have a very stark or industrial feel to the interiors. That’s not true at all here with upscale touches in the main living space, bathroom and bedroom. These spaces are a very pleasant place to spend time.
- For 2025, Jayco moved the furnace vents to the cabinets, off the floors. This means you won’t be kicking in dust and debris all season for that very special odor when the furnace kicks on as the temps go down.
- Like most fifth wheels, this one features an upper deck with a private bedroom. But there are also two drop-down beds in the cargo area on a power lift mechanism. One of those can go from opposing couches to a bed quite easily. This cargo area bedroom/living space also has a half bath. So, it’s a great place for kids.
- Norco’s newer Exact Slide mechanism is unique. The system is designed to support using the space even when the slide is in. This is unusual as almost all towable RVs specifically recommend not going into the slide when it’s retracted.
- There is a provision to install both rear- and side-view cameras in this rig, but another spot for an observation camera in the cargo bay. You could use this to keep tabs on the things you’ve loaded as you travel down the road. Or you could also use it to keep an eye on rambunctious hooligans sleeping in the cargo hold. You know, the “quit teasing your sibling” kind of hooligans.
Challenges
- There is a sewer dump port right under the main slide room on the road side. This will require you to undergo a bit of contortion to get to the gate valve. (It’s also could potentially cause an abundance of foul language — prompting you to have to buy one of those “I’m sorry for what I said in the campground” shirts.)
- Common to a lot of fifth wheels, the vent for the stove is so high on the wall I doubt most people will open it. I would like to see the return of the type of vent where you can open/close it from the inside of the trailer. Leaving this open will likely cause the flap inside to eventually blow off.
Best For
- Travelers who want to bring large toys like side-by-sides, dirt bikes and that sort of thing will love this. The Seismic 399 offers storage space and cargo carrying capacity to accommodate those types of things. But there are also plenty of places to bed down for the night and great off-grid functionality. So, this would be a great accomplice for those who love off-roading or trail riding.
- RVers who travel extensively with a work or hobby that needs a lot of space will love the amount of space the Seismic 399 provides. For example, we have a friend who does chainsaw wood carving. (Yes, that’s a real thing.) The Seismic 399’s large cargo bay would be a great place to work his trade and stash his tools, while the rest of the trailer would still provide nice accommodations.
Key Areas of RV Trader Reviews
RV Trader asked RV shoppers what was most important to them in their RV buying process. Potential buyers — like you — helped us to identify a list of six key areas that camping and travel enthusiasts always look at when they’re searching for the perfect RV: sleeping space, bathroom, living area, four-season capabilities, and storage space.
The RV Trader team then got to work providing you with an honest and unbiased evaluations. And that’s where I come in. I’m Tony Barthel, RV expert and enthusiast. I provide an honest look at RVs from an RV industry insider perspective. That way, you can learn the good, the bad, and the intriguing about any RVs I review. I inspect each RV with the buyer’s perspective at the forefront. And I always evaluate those six key areas that buyers have told us that they have in mind.
So, that said, let’s take a closer look at the 2025 Jayco Seismic 399.
Sleeping Space
The bedroom on the upper deck (common with many fifth wheels) features a queen-size bed in a bed slide. The nose of the Jayco Seismic 399, at the front of the bedroom, features a full closet. There is also a secondary closet that is plumbed to accommodate a stacking washer and dryer.
On top of all of that (literally), there’s a loft in the living space that a younger traveler would probably absolutely love. (It would likely be a fort by day, bedroom by night.) If you don’t have such a youthful camper with you, it’s easy to use this to store things as well. If you have more people and need more beds in the living area, there is also a fold-out bed as part of the large L-shaped sofa. But you may not need, because there’s more sleeping space in the garage.
About That Garage (Photos Under Storage Space)
In addition to the bedroom, loft, and foldout sofa, there are two larger beds in the cargo area that drop down by power-lift mechanisms. One of those are two couches by day and a bed by night; the other is a large proper bed. That last bed would be a good spot for a couple of adults to sleep comfortably. The folding couch/bed below does have some padding design that serves as a better couch surface, but it could provide a lumpy sleeping surface. (Which may be fine for younger campers, but adults might not be so thrilled with this.)
Overall, there are beds for nine individuals, and I could see at least four of those places being quite accommodating for full-sized adults.
Bathroom
The Jayco Seismic 399 follows the pattern found in most fifth wheels with a bathroom upstairs that incorporates a large, pleasant walk-in shower. There’s plenty of space around the toilet, and the bathroom is large enough that you could close the two doors that access it and change clothes. The two doors in this bathroom have one entry point into the upstairs bedroom and a second one into the hallway.
There’s a second half bath downstairs in the cargo area that features a toilet and sink. Additionally, there is secondary plumbing in this area to accommodate a washer-dryer, which means there are two washer-dryer hook-ups. Perhaps one job possibility is a laundry service at RV parks, although the intention is probably to only hook up one laundry pair, not two.
Living Area
This is another area this trailer really shines in the Jayco Seismic 399 with a plush-feeling L-shaped sofa. That sofa has two reclining positions which face the entertainment center and electric fireplace. On the road side, the portion of the sofa that faces directly into the camper is the one that folds out to a bed. The upholstery on this has a really nice feel. Jayco has assured us that it’s safe for this kind of RV being treated to minimize the chance of staining.
I really like the reconfigurable table system in this trailer. It can go from a short two-person table to a longer one able to easily accommodate five people. Rather than having leaves it has hinged sections that pull out with the table, which is an easy process.
Kitchen
Like the rest of the living space, the kitchen in the Seismic 399 portrays an upscale vibe. The kitchen itself is a U-shaped affair which results in a lot of usable counter space. There’s a three-burner propane oven and plenty of drawers and cabinets. For 2025, Jayco has upgraded the drawer builds on these trailers to a tongue-and-groove style build which is a much higher-quality way to put drawers together than what is common in most RVs.
I also really like the sink they chose, which features an integrated glass washer and a number of drop-in pieces that can be used to dry or clean dishes more easily. Further, one of these drop-in pieces is designed to work with a waterfall-like sprayer to wash veggies. Of course, the faucet incorporates a sprayer and pull-out head as well. This is quite literally more useful than the kitchen in my own house (and gives me ideas for remodeling).
I really enjoy these pull-out pantries like the one in here. This increases the amount of storage by using the depth of the pantry.
There’s a large 17-cubic-foot, 12-volt refrigerator in here, which is as large as many residential units. But it has an additional advantage: It’s able to run without kicking on an inverter.
Something unique to Jayco products is called the JayPort, which is a two-inch receiver hitch but in the side of the trailer. Jayco includes a flat-top propane griddle with this trailer that mounts in this place. This is nice and sturdy but also positions the griddle away from the body of the camper. Nice.
Four-Season Capabilities
The Seismic 399 has undergone zero-degree testing in the Dometic freeze chamber, proving you can camp safely and comfortably all year round, even in snowy conditions. Taking it one step further, the Seismic also undergoes weather chamber testing from zero to 100 degrees Fahrenheit. With Climate Shield zero-degree tested weather protection, the Seismic 399 has a fully-enclosed and heated underbelly, double-layered fiberglass batt insulation, and double-sided, bubble-foil radiant barrier insulation for best-in-class results.
Storage Space
One of the biggest reasons to get something like this is because of the huge garage in the back, which is absolutely a component of storage space. But Jayco has added a decent-sized front passthrough space in here as well. Other than that, there isn’t much exterior storage.
Inside there are plenty of cabinets and drawers for the kitchen and the front bedroom. There is a loft accessible above the kitchen which could be a great sleeping spot for a younger traveler but could also be used for a lot of storage if you don’t have such a person with you.
Closing Notes on the Seismic 399
It seems pretty clear to me that Jayco has really been paying attention to what customers are saying with the attention to detail and number of improvements to the Seismic line for 2025. While some of these changes, like the dovetail drawer construction, might seem minor they will make a difference in the long run on this RV.
As a professional looker-atter of RVs, I see a lot of companies just plodding along, but it’s clear that Jayco is making investments in improving the quality and usability of RVs like this.
Now, this is way too large an RV for many campgrounds, particularly on public lands. And this is a very heavy rig. So, something like this won’t serve all campers, but if a large toy hauler is what you want, I think you’d be mistaken not to consider this one.
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