I will give a firm X2 on Theodore Roosevelt National Park.
We camped in the park, but the sites are pretty small and no hookups, but that was OK with us. I can only comment on the fall (September) because that is when we were there, but it was beautiful and so NOT CROWDED like Yellowstone was even in September. We loved it.
Also, explore the town of Medora. See the play they put on. Eat at the outdoor steak fondue (steaks done on the ends of pitchforks!). Visit the Harold Schafer museum. You may not know the name, but you will definitely recognize the products. He also invested heavily in the town of Medora and help make it the lovely little town it is today.