2018 Grand Design Momentum 399TH RV for sale. This RV has all the upgrades and is in fantastic shape. It has been our full-time home for the past year and while we loved it, it's time to put down some roots. The RV has been redecorated to feel more open and comfortable. Video walkthrough, and additional details available upon request. The 2021 F350 in the photo is also available.