We just got back. Sylvan lake lodge campground wasn't bad. In September it aught to be quite nice...it's only down side is that it's sites were odd shaped and the people next to us were close....we were in site 6. I'd probably close another if I went there's again.
It sits right on the head of needles highway, that makes Rushmore very easy to get to, and scenic.
Be warned, it is at elevation...so if you have breathing problems. It might be worth looking elsewhere. It's at 6000 feet...didn't bother me, and I'm your average overweight middle age guy. Also note, the first real grocery store in custer...it's a little drive...but after a while out there, 30 minutes feels like a drive to the corner store

We did look at one other state run park, I can not recall it's name. It was off the road that goes east out of Custer, past the lakes..but it's not on the map :S
The commercial places around there are pretty bad. At least the ones we previewed. As is typical, they pack you etc.