We've stayed at Fishing Bridge. Very cramped, very very cramped. We also stayed in Bridge Bay which is not far from Fishing Bridge. That's where we will always now go. No hook ups but a really great central location with lots of wildlife.
In the Mt Rushmore area, stay at Rafter J RV park. Great place with lots of open space and your neighbor is a nice distance away. Also the road to Mt Rushmore is just across the highway so less than 3 miles. Crazy Horse is just 6 miles down the road. Take the 1890's train from Hill City, 3 miles away, to Keystone. Its a blast. Train lets you off right in the town of Keystone, just walk across the street and your there. Be sure to do the 16 mile drive with 30 some curves, 3 cross over bridges and much more.
Be sure to stop at the Little Big Horn Battleground. Very educational.
Ken Cash