Yellowstone CampingThere are 12 campgrounds inside the park. Five are run by concessionaire Xanterra and can be reserved.
There are fully hookups at Fishing Bridge RV Park - 300+ spots - it is an enhanced parking lot type facility. Some tables - but rigs close together. A bit expensive. Not really the place for a campground in the mountains experience - but some folks like it as a home base.
All the other campgrounds in the park are dry camping - no hookups.
The reservation CG are Madison (278 sites) near the west entrance, Grants Village (430 sites) at the south part of the park near the south end of the lake, Bay Bridge (432 sites) near the center of the park - west side of the lake and Canyon (273 sites) near the fantastic canyon.
Madison, Bay Bridge and Grants have sites for most any size rig - however, you need to go to the Xanterra site and check because there are not a lot of large size rig spots. And they go quickly. Reserve early.
The other seven campgrounds are run by the National Park Service - no reservations - first come first served. Mammoth is the only one open year round, and you must enter the park from the north in the off season to reach Mammoth.
You need to get to one of the NPS campgrounds very early - 9-10 am - to have a pick of good spots.
Tower Falls is size restricted to less than 30 feet, and Lewis Lake is 25' or less.
Recommend you look at the campgrounds with Google Earth or satellite map web site.