With only 6 days, coming from Cleveland, my suggestion would be to plan to spend all of that time in South Dakota. It'll take you two days to get there, two days to get back, leaving you the 6 days for relaxation.
You can visit Rushmore, Deadwood, Custer State Park (huge!) The black hills, the badlands, Wind Cave National Park, maybe a day trip over to devil's tower. Your itinerary will be completely full just in this small area, which is a very beautiful part of the country.
Best of luck in your planning! Just remember, a truck drives a lot faster than a truck pulling a 5er. Plan on averaging about 50 MPH with stops.