Hi, Happy--
The midpoint of your trip is near ND oil country, so reservations are a good idea. I've enjoyed Fort Abraham Lincoln State Park in Mandan, ND, on he bluffs above the Missouri R. It's about 10 miles off the interstate, so the Bismarck KOA may be more convenient. Makoshika State Park in Glendive, MT, is also pretty, though it's a haul from St. Paul. Medora, ND, is close to a midpoint for you. My preference there is Teddy Roosevelt National Park CG nearest to town or the municipal park CG. Buffalo roam freely in the NP, including in the CG, heads up.
