There is a campground just past (East side) the Custer Battlefield. Probably been 8-9 years since we stayed there but it was ok back then. Nothing special but it was just a place to stay that was off the road. Real reason we stayed there was to visit the memorial there. It is really neat in late afternoon early evening. Even if you don't want to camp there you should check out the battlefield.
Oh yeah you will find out that I 90 through Montanna is a real long drive.
Tom Marty