Between Rushmore and Yellowstone if you can take a short jaunt off the interstate and see Devil's Tower it's pretty cool.
And I know you said no hills, but if you can do the Beartooth Pass on the north east entrance to Yellowstone, to say the views are breathtaking is the understatement of the century. But it's a steep, narrow, windey pass that climbs to 11,000 feet. Motorhomes and rigs like yours do it all day long, but it's certainly not for the faint of heart. I do it a couple time per year with my rig and usually with a small trailer in tow.
