As mentioned, don't get started onto the Needles Highway in the Black Hills with your coach... see photo. We quite frankly see Yellowstone and Mt. Rushmore as too touristy anymore and avoided them the last big trip. That doesn't mean you should if you haven't seen them for awhile.
Consider dry camping at Elk Mountain Campground near the headquarters of Wind Cave Natl. Park, and toading through the backroads there. The campground has only a few sites large enough for our big units, so try to arrive there at an optimum time of day for finding spots. Jaunt south to Hot Springs to the wild horse sanctuary nearby, and the Spanish Mustang Ranch to the east. From Wind Cave head north to Custer and put in at the Beaver Lake RV Park on the highway west of town; call well ahead for reservations. Say Hi to the owner for me; he's a great guy heavily involved with the Good Sam organization, and who carefully and expertly monitors his very active park. He helped temporarily repair our awning when it got stuck in the out position, spending a good deal of time in the process, and has a well equipped workshop he allowed me to use.
From his location you can readily visit via toad the Crazy Horse Monument and access the Needles and Custer State Park, and Custer has all the banks, provisions, hardware, and etc. you could want. The only place I can't recommend is the Natl. Museum of Woodcarving - it looks enticing but we wouldn't go again.
I would recommend though the Little Bighorn monument if you can jog up that way. The 7th Ranch is another excellent nearby RV park with great hosts, and you may not have been to the Monument since the Indian memorial was installed. Whatever else you do there, don't miss the van tour by the Crow Tribe.
The one issue we find with this country is the frequency of fires. We left Custer just as a big one flared up near where we'd been at Elk Mtn. Campground, then had to drive through stifling Wyoming smoke just to get from Custer to Sheridan. It was hard to find an area where smoke wasn't an issue, but that's just part of traveling the West in midsummer.
Joel