A Great Article From rv-camping.org:

Yellowstone EntranceYour Yellowstone RV camping adventure will include seeing abundant wildlife living among the world’s greatest concentration of geysers. Yellowstone National Park has 12 developed campgrounds with over 2100 campsites. 5 reservation campgrounds have 1700 campsites, and the remaining 7 campgrounds have over 450 first come first serve campsites. Some large RV pull through campsites are available by reservation, and first come first serve.

All Yellowstone campsites have a fire ring/grill and picnic table with the exception of Fishing Bridge RV Park. Water, flush toilets, and trash receptacles are provided, and RV dump stations are available at some campgrounds. Popular first come first serve campgrounds may be full by 8AM. Most campgrounds will accommodate RVs 30 feet in length or less.