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.