There is some disbursed camping with a fee located between Yellowstone and the Tetons national parks (John D Rockefeller Parkway). Not as rustic as I PREFER for disbursed camping but about as close as you'll come in that location. There are some rustic CAMPGROUNDS operated by the NFS just North of the town of West Yellowstone and that's a pretty good place to tour MOST of yellowstone park from. There are other NFS campgrounds south and west of Glacier National Park, along the east side of Flathead Lake. I don't give out the EXACT location of my favorite boondocking sites but with a little effort you can find them in those areas. For what it's worth, we're 34 feet long (56 when hitched) and have used all of those sites with our trailer.
Good luck / Skip