We stayed for a month in Custer, SD this summer (Aug/Sep). So much hiking and touring we did in the area including Custer State Park, Needles highway, Rushmore, Berglum Museum, Devils Tower, Badlands, Walldrug, fishing Sylvan Lake, Deadwood, and hiking to the top of Harney Peak. We still missed out on things like Crazy Horse, Spearfish, Buffalo roundup, Emerald and Wind caves, Hot Springs, and the Hill City train. Just not enough time to do it all.
We stayed at Beaver Lake Campground. It is a really nice campground 4 miles west of Custer. We chose the campground based on internet reviews and we were not disappointed. The property is tree covered with large ponderosa pines. The sites and roads are gravel and well maintained. As a bonus, they have lots of semi-tame rabbits that come around mooching for animal crackers which they sell for a quarter in the front office. Some folks had problems with their cell phones but with our booster we were able to make calls and get internet (4G through Verizon). For the most part the weather was awesome (in late Aug to late Sep). High temps in the mid 70's with lows in the 50's although we had one three day cold snap in early Sept where we got snow and the low was 26. Melted off the nextday. That is unusual weather but added to the experience.
The area is a must see and we will be going back again.