Can't really add to what the weather will be like...we stayed in Hermosa(Hwy 79) for the 7 days we were in the area and the log states: 79 degs.(June 21, '10)
We stayed at Heartland RV (clean, full H-Us and nice people),because of the central location to Badlands NP, Wind Cave NP, Custer SP, Needles Hwy., Iron Mountain Rd and don't miss Crazy Woman Canyon(hwy 33...ask the locals for directions) and of course, Mt. Rushmore.
The CG and Hermosa for that matter, isn't much to write home about but boy, is it in the middle of all the attractions. We stayed two extra days to travel (toad)all the roads in both directions and could have stayed longer.
Very interesting area.
Saw most of our buffalo at Wind Cave NP on a SE back door dirt road into the park...we were lost and had no idea how we got there. We did end up at the cave and found it refreshing from the hot sun. The whole time we were in the park, I thought "I had been born two hundred years too late"...it's beautiful and hope to revisit sometime. You'll want to spend as much time in the area as possible.
Have a great time. Bob