One of the most scenic spots in S DAkota is Custer State Park. You pay for a pass to access the park but so worth it!
There are three loops to drive where you can see lots of wildlife(buffalo and mores), rock spires and unique pigtail (curling) road. One of the drives brings you through a one way tunnel where Mt Rushmore is framed by the low/tight one way tunnel. Be sure to check size restrictions for that loop. NO RVs are allowed and I do not think a truck camper would fit the tunnels.
S Dakota is beautiful. Might check out Wind Cave and the other cave (name escapes me). Know you will enjoy it.