Rapid City, SD area RV park

In June we stayed at Custer's Gulch in Custer that would fit the bill I think. When we arrived on a Saturday, Rocky Mountain Elk Foundation was having a meet up there. Mostly Full hookups (some electric only, but they do have a dump station on site) and sites of all sizes, mostly back in but long, a few pull throughs, The sites are spaced out so you're not crammed in like you would be at the KOA and many other parks in the area. Clubhouse, and some cabins too. Only caveat is there isn't much for kids to do if that's a concern for your group.

Rafter J Bar looked really nice too. Just was more expensive than Custer's Gulch, probably due to having a pool etc.