I've heard that driving the Needles Highway is quite difficult with most any RV due to its narrowness and tight curves; probably OK with a truck camper or a Class B. As agesilaus pointed out above, the visit at Crazy Horse is about 3 hours, and maybe a little longer at Mt. Rushmore (especially if you are parked a long way out). The drive over to Devils Tower is pretty nice and you've got it figured just right. You may want to include a trip to Badlands NP, but possibly you could include that on your way to the Black Hills from back east.
