The 2026 Grand Design Momentum 399M is roomy enough for two adults to live in comfort and flexible enough to host a small crew.
That said, there are definite design “hits” and “misses” in this RV. Read on and see if you agree.
Targeted buyer
The Momentum 399M for 2026 is best for couples who want residential comforts plus a good-sized garage to carry their golf cart, motorcycle, electric bikes, or other “toys”. Couples who occasionally host friends or run a remote office will like the layout, as well.
Behind the walls
Grand Design built the Momentum line with a focus on insulation and structural durability. It features an aluminum-cage frame, fully laminated walls, and a thermal roof assembly. There’s also a multi-layer insulated garage floor to keep the toy bay usable in a wider range of temperatures. According to the salesperson I talked with, “The company is listening to its customers.”
The 399M features three slideouts that really open up the main living space. Designers fit many elements into this toy hauler—some I like and others—not so much.
Key specs
• Overall length: 44 ft. 10 in.
• Height: 13 ft. 6 in.
• UVW: 19,170 lbs.
• Dry weight (approx.): 18,000 lbs.
• GVWR: 23,500 lbs.
• Hitch weight: 3,464 lbs.
• Fresh water capacity: 140 gallons
• Gray water capacity: 104 gallons
• Black water capacity: 52 gallons
• Garage length: 12 ft. 6 in.
• Standard bed: 60 x 80 (queen)
• Number of slideouts: 3
• Number of awnings: 3
• Refrigerator: 20 cu. ft.
• Electrical service: 50 amp (solar/off-grid options available)
Typical pricing on the 2026 Momentum 399M runs from the mid-$150,000s to about $195,000 (before tax and fees), depending on dealer. MSRP is $174,278.
Interior and living space
My first impression when stepping into the Momentum 399M: “That ‘M’ must stand for massive!” The slideouts and high interior height deliver a bright, airy feel that seems very residential—almost like an upscale apartment.
All of the materials and finishes are aimed at a “higher level,” according to the salesperson I spoke with. You can judge for yourself in the video walk-through at the bottom of this post.
Kitchen
The kitchen is designed to make cooking for two as easy as preparing for a group of friends. However, it could get a bit crowded if more than one person wanted to help. Here are some of the features I liked, along with some drawbacks:
An integrated sink strainer is helpful; however, the large sink eliminates valuable space for counter prep.
The rig’s large pantry has helpful pull-out drawers, but access to the extended drawers is tricky.
The peninsula-type layout makes the area behind the sink feel tight for more than one person.
The Momentum 399M comes with a spacious three-door refrigerator, but when the fridge doors open, you’re backed into the peninsula. A “hit” and a “miss” for me.
The large range and oven are great for folks who entertain. I wish the microwave were a combination micro/convection, instead.
Cupboards and drawer storage are positioned in sensible locations around the range and are plentiful. A design “hit” for sure! The “miss”? Positioning charging ports at the back of the drawer!
Dining area
The dining area/workspace was a design hit for me because I write while RVing. I like the versatility of the space, and the large windows will let in lots of light and pleasant breezes! Definitely a “hit.”
Living room
Seating and entertainment in Grand Design’s Momentum 399M prioritize socializing. The standard layout places the TV and media center where a small group can lounge. The RV I toured featured a sofa and two recliners. There was adequate seating for four as long as the recliners remained upright (not reclined). However, viewing the TV while sitting on the couch could become uncomfortable.
The hidden storage behind the living room’s TV delights some buyers. For me, it’s a gimmick. I’d rather have the extra cabinet space in the garage.
The optional side-patio extends living space outdoors and is outfitted with a griddle, sink, small refrigerator, and gigantic TV. I like the idea of a patio, but wonder if we could use this feature in narrower campsites.
Between a fold-down sofa and a bed that lowers from the garage ceiling, you’ll have space to sleep four in the garage. Fido will discover a built-in feeding station in this space, and the rear patio is spacious enough for two to watch the sun go down. A “hit” for me.
Bath
The bath in the 399M aims for a residential feel. I appreciate the roomy shower, integrated baskets for shampoos, and fold-down bench.
There’s adequate space around the commode and plenty of storage for linens and towels. However, like in the kitchen, accessing the storage space is a bit problematic. It felt awkward to me.
The sink features good counter space and even a bit of undercounter storage. The lighted mirror is a luxury touch, but I’d prefer an actual medicine cabinet.
Bedroom
The front bedroom in the Momentum 399M is impressive because of the storage space. The rig comes with a queen bed (with an option for a king). You have space to walk around the bed and charging ports for your devices, but no nightstands on either side of the bed. Ugh.
I’m not a fan of the headboard style. It features a guaranteed head-thumper shelf fastened to the top. I like to sit up in bed to read, and the shelf would need to be removed to allow that.
Closet and wardrobe storage is sized for extended trips. I love all of the drawers and (finally) easy access. The bedroom’s position helps isolate sleeping space from party or garage activity—a definite plus for me.
Overall take
If you’re a couple who wants a real-home feel with legit toy hauler capability, the 2026 Momentum 399M might work for you.
I appreciate many of the features in this unit and understand what the designers hoped to achieve. However, if they lived three days in this RV, I think they would see areas for improvement. Watch the video below or tour a Grand Design Momentum 399M for yourself to see if you agree.
Thoughts? Tell me in the comments below.
To learn more about the 2026 Grand Design Momentum 399M, click here.
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RVT1245b
Too big, too tall, too pricey
Wow! Which new campgrounds have the space to park it, or the narrow roads to the campsite?