You shouldn't have any problems finding space after Labor Day along I-94. There's a KOA along the interstate in Jamestown. A KOA in northeast Bismarck that's decent but will still be quite busy. General Sibley Campground (County Park) in south Bismarck with lots of room and lots of shade. Ft. Lincoln State park south of Mandan which is peaceful and excellent and won't be busy after Labor Day. Then Medora and Theodore Roosevelt Natl Park in western ND - primitive campground in the park, 2 ok campgrounds in Medora (may want to call to make sure they're still open depending on how long after Labor Day).