On the way to YNP, Mt Rushmore and the Black Hills would be a great addition to your adventure if you have not already been there. The Needles Highway is an awesome drive, assuming your vehicle can fit thru a 8'4" x 10' tunnel.
Just returned from there - highly recommend Grizzly RV Resort in West Yellowstone, MT.... One of the best CG's we stayed at on our month long trip... Only abt a mile from the West entrance to the park and walking distance from town (abt one block)