Do be aware that the seat belts installed in the rear seats of motorhomes are often not up to the standards of, say, passenger car seat belts. They sometimes have less than excellent mounting structures and general designs; and also some of the seat designs themselves are not really sturdy enough to be particularly crashworthy. For sudden stops or comparatively minor collisions, they would doubtless help keep people in the seats and prevent some injuries. In the event of a serious accident, though, I'm not sure they would be of a great deal of value. Then again, a motorhome is not generally a great place to be in a serious accident, between the many potential projectiles, the edges of furniture, and the typical lack of beefy structure.
My '98 Coachmen has a total of nine positions with seat belts: two in the cab area, four around the dinette, and three along the couch. The ones in the back are only lap belts, and truth be told I'm not sure they've ever been used, either by me or by the previous two owners.
My RV also technically has four wheel drive, although admittedly all four driven wheels are on the same axle... :B