Regarding Teton - we stayed at Coulter Bay in 2006 with our 28' rig, and it was tight. Diesel was $5/gal at the park station.
In Yellowstone, we stated at Fishing Bridge. A little tight, but a good base of operations to tour Yellowstone.
Someone else asked about Mt. Rushmore in S. Dakota -
Big Pine Campground which is a little west of Custer, or Custer State Park. We stayed at Big Pine in Sept '07.