We stayed at Buffalo ridge RV campground and resort in Custer (the city). It's a Good Sam campground, so if you are a Good Sam member, you'll get the 10% discount. Theirs are all full hook-up. The location is good, easy access to just about everything. More than likely you won't be spending much time at the campsite anyway, there is just simply too much to see and do, everything from Sturges to Deadwood to Needles Highway, to Mt. Rushmore, to the Badlands, to touring around the Black Hills. You can also rent ATV's and go, go, go! Way too much to do than sit around a campfire. You can do that at home! Use the campsite as a home base, but I doubt you'll be "home" very much, except to sleep.
Anyway, good luck. You'll enjoy the area, tremendously. (FYI, we were there 2 weeks and still did not see everything we wanted).