We traveled to Yellowstone in September. On the way we stayed at Custer State Park, Game Lodge campground. They have sites large enough for a 40' motorhome. They have 50 ampere service, but no other utilities on site. The first morning, there was a herd of bison passing by the campground. We used this as our base for Mount Rushmore and Crazy Horse. The State Game Lodge is adjacent to the campground and has a good dining room. Custer State Park itself is well worth exploring. Many of the roads through the state park are not suitable for a large RV because of the tunnels.
While visiting Yellowstone, we stayed at Yellowstone Grizzly RV Park which is located in West Yellowstone, Montana.
Don't miss the Grand Teton National Park, which is just South of Yellowstone. While there we stayed at Colter Bay Village RV Park. We took a Jackson Lake Cruise, which was outstanding.