We stayed in the Legion Lake campground (sites 3E & 4E)last summer. We have a 30 ft. tt and our friends who traveled with us have a 35 ft. fw. No problem for either of us getting in/out of our sites. Legion Lake is right across the road from the campground, very scenic and kids did swim there. There is also a small store at the lake. The nice thing about Legion Lake CG. is it is very centrally located in the park,and easy to navigate to any location from there. Very clean bathroom/shower facility.
So much to do there. Check out the buffalo/burros. Hike to Harney Peak. Drive the Needles Highway (without tt) and take Iron Mountain Road out to Mt. Rushmore- these are both breathtakingly beautiful roads! Don't forget Crazy Horse Mem., Jewel Cave, Deadwood for a reenactment ect. Oh, and don't forget a stop at Wall Drug if you are coming from the east- pretty cheesy but you can say you've been there/done that.
Have a great time, you will love it!