01 What makes the 2027 399 different
The Seismic 399 is one of Jayco's Seismic fifth-wheel toy haulers, the flagship of the Seismic range, built around a rear garage behind the living space.
Across the 4 model years of the 399 on this site (2024 to 2027), the 2027 record has the heaviest dry weight, 15,585 lbs; the highest GVWR, 21,000 lbs; the smallest cargo capacity, 5,415 lbs; the lightest dry pin weight, 3,265 lbs. It is the latest of them.
Among the 4 Jayco Seismic fifth wheels with a 2027 page on this site, the 399 is 2nd lightest of 4 by dry weight; joint highest of 4 by GVWR; the largest of 4 by cargo capacity. By dry weight, the lightest is the 359 at 13,490 lbs and the heaviest is the 395 at 16,075 lbs.
02 Full 2027 specifications
Dimensions
- Exterior length
- Not published
- Exterior width
- 8' 6"
- Width, slides out
- 13' 6"
- Exterior height
- 12' 6"
- Exterior height (w/ A/C)
- 13' 3"
- Interior height (main)
- 8' 3"
- Garage length
- 13' 6"
Weights
- Dry / unit base weight (UVW)
- 15,585 lbs
- GVWR (max loaded)
- 21,000 lbs
- Net cargo / payload (CCC)
- 5,415 lbs
- Dry pin weight
- 3,265 lbs
Running gear
- Tire size
- ST215/75R17.5'H'
- Hitch type
- Fifth-wheel (in-bed)
- Model year of record
- 2027
Capacities
- Fresh water
- 106.0 gal
- Grey water
- 87.0 gal
- Black water
- 87.0 gal
- LP / propane
- 60 lbs
- LP / propane, optional
- Not published
Galley & systems
- Water heater
- Tankless
- Furnace output
- 35000 BTU
- Furnace, auto-ignition
- Not published
- Outside storage compartments
- 2
Sleeping
- Sleeps
- up to 8
Drawn to scale
Weight budget
Holding tanks
Axles, wheels and nose shape are schematic; not a floorplan. Data: dataset.
Towing it
Loaded range: 15–25% of GVWR on the pin, a rule of thumb, not a measurement of this unit. No truck ratings or hitch classes (why; NHTSA, DOT HS 809 433). The range starts at the published dry figure, because loaded is never lighter than dry.
03 What changed for the 2027 model year
Between the 2026 and 2027 399, 8 of the 19 fields both records publish differ: dry weight, GVWR, cargo capacity, dry pin weight, exterior height, height with A/C, tire size and water heater. The other 11 match.
| Field | 2026 | 2027 | Change |
|---|---|---|---|
| Dry / unit base weight (UVW) | 14,950 lbs | 15,585 lbs | +635 lb |
| GVWR (max loaded) | 20,400 lbs | 21,000 lbs | +600 lb |
| Net cargo / payload (CCC) | 5,450 lbs | 5,415 lbs | -35 lb |
| Dry pin weight | 3,310 lbs | 3,265 lbs | -45 lb |
| Exterior height | 12' 8" | 12' 6" | -2 in |
| Exterior height (w/ A/C) | 13' 2" | 13' 3" | +1 in |
| Tire size | 215/75R17.5H | ST215/75R17.5'H' | — |
| Water heater | 16 | Tankless | — |
Published for one of the two years only: exterior length (2026: 46' 9").
| Model year | Dry (UVW) | GVWR | Dry pin | Sleeps | Tire size |
|---|---|---|---|---|---|
| 2024 | 14,560 | 19,995 | 3,395 | 9 | 215/75R17.5H |
| 2025 | 14,950 | 20,400 | 3,370 | 8 | 215/75R17.5H |
| 2026 | 14,950 | 20,400 | 3,310 | 8 | 215/75R17.5H |
| 2027 | 15,585 | 21,000 | 3,265 | 8 | ST215/75R17.5'H' |
All years side by side: the Jayco Seismic 399 parent page.
Where these numbers come from
Every figure on this page is read from Jayco's own 2027 model page for this floorplan (how sources are handled).
Fields Jayco does not publish for 2027
For this model year the factory record has no exterior length, optional propane and auto-ignition furnace. Those rows read “Not published” and are not filled in from another year.
04 Buying a 2027 399 today
Its age, and the date on its label
Model year 2027 runs ahead of the 2026 calendar year. Its build month is on the certification label (49 CFR 567.4(g)(2)), not in the model year.
Tires: size and date code
Jayco lists ST215/75R17.5'H' as the 2027 factory tire size. Other years on this site list 215/75R17.5H (2024, 2025 and 2026), so a replacement set has to match this year's size. Read the four-digit date code at the end of each tire's identification number, spare included (49 CFR 574.5(b)(3)).
NHTSA record for model year 2027
The federal record on Jayco Seismic year by year runs from model year 2005 to 2026, so it holds nothing for 2027 yet.
05 How it compares — and to its other years
Differs from 2027 on 8 of 19 shared fields, dry weight, GVWR and cargo capacity among them.
06 Loading it for a trip
A full 106-gallon fresh tank is about 884 lb of water (water counts as cargo: 49 CFR 571.120 S10.4.2), 16% of the 399’s 5,415 lb cargo capacity, leaving 4,531 lb (566 lb per rated berth). Your own unit’s figure is on the label inside its entry door (49 CFR 571.120 S10.4.3).
07 Where this floorplan sits
| 2027 fifth wheels from other lines, nearest by GVWR | GVWR (lb) | Dry (lb) | Sleeps | Length |
|---|---|---|---|---|
| 399 | 21,000 | 15,585 | 8 | — |
| Cyclone 3913 | 21,000 | 15,435 | 8 | 46 ft 5 in |
| Momentum 394G | 22,500 | 16,400 | 7 | 44 ft 11 in |
| Pinnacle 39FLOK | 18,800 | 15,920 | 6 | — |
| Cedar Creek 381MUD | 18,000 | 15,429 | 8 | 43 ft 5 in |
| North Point 365RKTS | 17,300 | 14,075 | 4 | — |
| Eagle 355MBQS | 15,400 | 12,775 | 8 | — |
Against the 2027 399: the 2027 Momentum 394G is 1,500 lb heavier and sleeps 1 fewer; the 2027 Pinnacle 39FLOK is 2,200 lb lighter and sleeps 2 fewer.
08 Common questions about the 399
How much does the 399 weigh?
15,585 lb dry; 21,000 lb GVWR, the most it may weigh loaded.
How much can the 399 carry?
5,415 lb, 26% of its GVWR (median across 307 fifth wheels here: 19%). A full fresh tank uses 884 lb of it.
What is the pin weight of the 399?
3,265 lb dry, 21% of the dry weight (median across 386 fifth wheels here: 19%).
What size tow vehicle does the 399 need?
A tow rating of at least 21,000 lb, plus payload for the loaded pin weight, passengers and cargo.
How many people does the 399 sleep?
It is rated to sleep 8.
What are the tank capacities on the 399?
Fresh 106, grey 87, black 87 gallons. Off hookups, fresh water runs out first.
How does the 2027 399 differ from other model years?
Set against the published 2026 399 record, 4 figures differ: dry weight 15,585 lb in 2027 against 14,950 lb in 2026; GVWR 21,000 lb in 2027 against 20,400 lb in 2026; cargo capacity 5,415 lb in 2027 against 5,450 lb in 2026; dry hitch weight 3,265 lb in 2027 against 3,310 lb in 2026. 4 other figures both years publish are identical. This floorplan is profiled for 4 model years here (2024 to 2027).
From the figures above.
09 What this page publishes, and what it does not
12 of the 18 fields this catalogue tracks are published for the 399. Not published: exterior length, slide count, axle count, awning, refrigerator, air conditioning.
Sourcing rules: methodology.
10 Manuals & documents
Sources: jayco.com (linked below); the full source note is at the foot of the page.
11 NHTSA safety record
NHTSA holds 17 recall campaigns and 13 owner complaints against the Seismic line. They are counted separately: a recall is a confirmed defect with a remedy, a complaint one owner’s unverified account.
What to look at on a used one. Across the 13 complaints filed on this line the most-reported components are suspension (38%), unknown or other (23%) and service brakes (15%).
Recall coverage here is not complete, and complaints are owners’ unverified reports (how this record is built). Check a specific VIN at NHTSA before you buy. Full tables are on the Seismic line page.