The 2026 Grand Design Momentum 399M is a luxury fifth-wheel toy hauler designed around flexible garage living, residential comfort, and extended-season durability. It’s built for RVers who want the ability to haul toys, create a second living space, or simply enjoy a large rear “flex room” while still having a high-end, apartment-style main coach. At approximately 44’10” in length with a dry weight around 19,170 lbs and a GVWR of 23,500 lbs, it sits near the top of the Momentum lineup for size, capability, and premium features.
Inside, the 399M is all about versatility. Instead of a traditional fixed garage, the rear area is designed as a multi-use flex space that can function as a garage, lounge, office, guest room, or entertainment zone depending on how you travel. One of its signature features is the rear “Party Patio” system, which converts into an outdoor entertainment deck with integrated appliances and a large exterior entertainment wall—effectively giving you a second living room outside the RV.
The main living area feels like a modern residential loft. With triple slide-outs, the center space opens up dramatically to include theater seating, a sofa, and a large entertainment center with fireplace. The layout is designed so that everyone in the coach can gather comfortably without feeling crowded, even on longer stays.
The kitchen is positioned as the heart of the coach, featuring a residential-style refrigerator (up to ~20 cu. ft.), 3-burner cooktop, oven, microwave, pantry storage, and a full countertop workspace, often with bar seating or a dinette that doubles as a work area. This makes the 399M especially popular with long-term travelers who want full cooking capability on the road.
Up front, the private master suite includes a queen or optional king bed, wardrobe storage, and washer/dryer prep, giving it a true residential bedroom feel. The adjoining bathroom features a large walk-in shower, porcelain toilet, and vanity storage, designed for extended use rather than weekend camping.
From a construction standpoint, the 399M is built on Grand Design’s heavy-duty Momentum platform, including 101” wide-body construction, laminated fiberglass walls, triple axles, and a fully enclosed and heated underbelly for extended-season camping. It also includes a 6-point hydraulic leveling system, multiple power awnings, and large pass-through storage, making setup and campsite living easier despite its size.
Reasons to Buy the 2026 Momentum 399M
- Unique garage-to-living flex space design
- Rear Party Patio outdoor entertainment system
- Spacious residential main living area with triple slides
- Full kitchen with residential refrigerator and pantry storage
- Private master bedroom with washer/dryer prep
- Heated and enclosed underbelly for extended-season use
- Triple-axle stability for heavy-duty towing
- Multiple living zones (indoor + outdoor + garage flex space)
- Strong storage capacity for long trips or full-time use
- Premium Grand Design build quality and resale appeal
Why Buy from Lazydays RV Powered by Campers Inn RV
- Large selection of Grand Design Momentum toy haulers
- Competitive financing options for qualified buyers
- Factory-trained RV technicians and nationwide service support
- Trade-in assistance for easier upgrades
- Full walkthrough and delivery orientation included
- Trusted national RV dealership group with long-term support
Key Features
- Grand Design Momentum 399M fifth wheel toy hauler
- Approx. 44’10” length, 13’6” height, 8’5” width
- Dry weight approx. 19,170 lbs
- GVWR approx. 23,500 lbs
- Sleeps up to 5 people
- 3 slide-outs
- 3 axles with heavy-duty suspension
- Rear flex garage / entertainment space
- Rear Party Patio system with outdoor entertainment features
- Residential refrigerator (up to ~20 cu. ft.)
- 3-burner cooktop, oven, microwave
- Theater seating + sofa lounge area
- Fireplace with entertainment center
- Front master bedroom (queen/king optional)
- Washer/dryer prep