Sidney, NE is a good place to restock your groceries, etc. before going north into tourist country. And we like shopping at Cabelas, too. There is a nice rv park at Cabelas. Custer State Park, IMO, is as good or better than any National Park. It is well run and has great scenery and the scenic drives are well worth your time.
Rushmore...not so much, but that's just us. Same in Devil's Tower...good for a one night stay. Buffalo WY has two or three rv parks in town that are good, but you could go on up to Ranchester WY and stay at a little mom and pop rv place in town and then take your toad up the mountain to Burgess Junction and the Medicine Wheel. It's a beautiful drive. You can get a sandwich, etc at Burgess Junction or take a picnic lunch and have it up at Medicine Wheel.