I would agree with 2gypsies about the Tetons. We stayed there at Colter Bay RV Park. It became one of our favorites.
On the way you could visit the Black Hills area of South Dakota, Custer State Park, and Mount Rushmore. Two or three days would not be too much to spend there. The drives through the state park and the Needles highway are worth the visit.
On our only RV visit of a few days to Yellowstone we stayed at Red Rock RV in Island Park, Idaho--a beautiful area, but about 20 miles outside Yellowstone National Park.
