You should unhook your rig about a mile inside the gate, just past the prairie dog town and on your left. It's where you'd turn to go to the campground.
There's an "over-run" parking lot near the visitor's center that buses and motor homes can use, but if your motor home has a toad or you're pulling a TT or 5er, you should plan on using the RV lot that's intended for them, especially now, during peak tourist season. As I recall it's all pull-through parking and all asphalt, less than a 100 yards off the main road. Do yourself a favor and use it. We tow a 26' 5er, and I'd never consider pulling it past the RV parking lot. It's just too small and crowded at the visitor center, and there's nowhere to turn around once you go past the turnoff for the campground.
The KOA is nice just before entering the gate, or if you can live without hookups, the campground adjacent to the trailer parking lot is nice -- one of our favorite campgrounds. I've posted this picture before on this board, but I'll do it one more time to give you an idea of the view from the Belle Fourche River campground. This was taken just a couple steps past a walk-through gate at the north end of the campground.
