The 2026 Grand Design Momentum 399M is a premium luxury toy hauler fifth wheel built for RVers who want a true combination of residential living space, large garage flexibility, and high-end extended-season construction. At approximately 44’10”–45’ in length with a dry weight around 18,000 lbs and a GVWR of 22,500–23,500 lbs, this model sits in the upper tier of the Momentum lineup and is designed for serious travelers who still want upscale comfort at camp.

Inside, the 399M stands out because it’s not just a traditional toy hauler—it’s designed as a “flex-living” platform where the rear garage can function as a cargo space, office, lounge, or additional sleeping area depending on your needs. The garage includes a large ramp door that converts into a patio deck, extending your living space outdoors and creating a second entertainment zone beyond the main cabin.

The main living area feels more like a modern apartment than a garage-based RV, featuring opposing slide-outs, theater seating, a sofa, and a residential entertainment center with a fireplace. A central kitchen island anchors the space, giving you real countertop workspace, while large windows throughout the slide-outs bring in natural light and open up the interior when fully set up.

The kitchen is built for full-time travel use, offering a large residential refrigerator (up to ~20 cu. ft.), 3-burner cooktop, oven, microwave, and deep sink with solid-surface countertops, plus a full pantry for extended trips. This makes it easy to cook for families or groups without relying on campground amenities.

Up front, the private master bedroom includes a queen bed slide (king optional), wardrobe storage, and washer/dryer prep, giving owners a true residential retreat. The adjacent bathroom includes a large walk-in shower, porcelain toilet, and vanity storage, designed for comfortable daily use even during long-term stays.

One of the defining strengths of the Momentum 399M is its dual-living-zone design, separating the garage and main coach so multiple groups can coexist without feeling crowded. The garage itself can include a half bath, additional sleeping options, and entertainment setup, making it extremely versatile for families, toy haulers, or mobile lifestyle users.

Built on Grand Design’s Momentum platform, the 399M features laminated fiberglass construction, a fully enclosed and heated underbelly, and heavy-duty triple-axle running gear, making it capable of extended-season camping and demanding road travel. Exterior highlights typically include , ensuring easy setup and strong campsite stability.