I will recommend Custer State Park in South Dakota. They have several campground located around the park. The prettiest on is called Sylvan Lake; located not far from Mt. Rushmore. They have several tunnels that are too narrow for a trailer to pass throught but there are other ways to get around.
The one we stayed at was called Game Lodge and it is located close to the wildlife loop road. You'll see lots of Buffalo and Antalope. Also Blue Bell campground is located close to the wild life loop road.
Custer State park is lots of fun. They also have several scenic drives that take you around the park and through the narrow tunnels on very winding up hill roads.
Check out there web site and be sure to make reservation far in advance. Very popular State Park.
My favorite places to visit are Yellowstone National Park and Custer State Park.
Tom