Firstly, two days is nowhere near enough to see the Black Hills area.
I suggest you find a place to stay in the heart of the hills, near Rushmore, to tour that area. We always stay at the Rafter-J-Bar
and highly recommend it. It's a short and scenic drive to Mt Rushmore from there, Custer State Park, which you must do and the Crazy Horse monument which we also recommend. We'll usually spend one to three weeks there when we visit.
While you could day trip to Deadwood (driving up through Spearfish Canyon) and even Devils Tower from there you'd probably be better off finding someplace closer to both, from an efficiency standpoint. Can't help you with that although I know there's a KOA within sight of Devils tower and at least one nice RV park at Deadwood.