Just a few suggestions. A good stop would be in Lexington, KY at the Kentucky Horse Park which is an RV campground and allows a visit to the Bluegrass region and Lexington itself. (Just off I-75)
We have stayed at the KOA in Sioux Falls, SD, just across the river from Sioux Falls, IA. In this area you are not too far from Pipestone National Monument. Not sure about camping there.
I won't go into the campgrounds around Sturgis/Custer except to say that we stayed at Custer's Gulch in Custer and found it convenient for most of the popular sites.
