We have been to South Dakota a couple of times. Mt. Rushmore is ok. It use to be better years ago before they commercialized it. It does not cost to see the monument but it cost you to park in their overhead parking garage.
If you are going to that area, stay at Custer SD and I would suggest
Custer's Gulch RV Park. Nice place good price and pretty quite. From there it is close to
Crazy Horse which has a fantastic light show at night. Get their early and park on the front row of the parking lot for a great laser light show. While your there go to the two scenic drives,
Iron Mountain and
Needles and Wilderness Loop.
If you go there and just see
Mt. Rushmore you will be disappointed. I have made some links so that you can see some photos of our trip in 2009.
Also if your around that area the first of August you might cruise through
Sturgis. Be sure to get a reservation before you go at this time of year.
Travel safe
BC
Then you can go to Mt. Rushmore which is not very far away.