We were in that area last month for 5 days. We stayed at Custers Gulch RV Park. It was very nice with friendly staff. We enjoyed Mt. Rushmore very much. There is a $10 charge for parking. ($5 for seniors) Everything else is free.
The Crazy Horse monument charges $11 per adult and something less for kids. You can see the monument from the highway. The visitors center there is okay, but they are trying to sell you something at every turn. We felt like it was a bit of a money grab.
Custer State Park was great. The cost is reasonable and the wildlife is wonderful to see.
We also drove up to Deadwood one day. We had a good time there visiting the casinos and restaurants.
Hope your trip works out for you.
Bill