Went in May, many years ago. Stayed at the Mt. Rushmore KOA:
https://koa.com/campgrounds/mount-rushmore/What we weren't expecting was freezing weather and snow in May! The week prior to our trip was much worse. The snow plowed off the main roads was higher than our full sized van.
Snow was starting to melt during the day, and still hovering around freezing at night. Campground was almost empty. We had a popup. I used the heatstrip in the A/C, and a space heater to stay "comfortable".
June or later would likely be better. This was a decent campground if you find camping at the parks full.
