You have good advice on campgrounds but none on time to tour the Black Hills. The only advice I will give you on that is do not try to do it in August. The Sturgis Motorcycle Rally takes place then and you will not find an empty RV park or campground nor at this late date will you probably be able to make reservations.
Believe me that I know what I'm talking about. We were fulltime RVers for 10 years and before that lived in the Dakotas for 21 years. We also had a membership in Hart Ranch RV Resort and even members had to have reservations in August or they couldn't get a site.