Spearfish SD has a city RV park that is FHU and reasonable. It is at the north end of Spearfish Canyon ( a drive you really need to experience) It will feel weird headed South from Spearfish because you are going up hill. The CGs in Custer SP have electric. They really need upgrading with water and sewer. A few don't eve haven a dump station.
Nicest is probably Game Lodge (more space, flatter, paved sites) Prettiest (IMO) is Blue Bell, set in pines and more scenic.
If I were staying from over a few days, I might go to a commercial CG with FHU for covenience