Depending on the month, camp grounds fill up fast in the mornings. Madison is the biggest one, about 15 miles in from West Yellowstone. If you want to stay at Fishing bridge, make reservations way early. Its like a little city there the only place with full hook-ups. Just driving past it turns me off. Think of main park road as a figure 8. Madison is at the center of the 8 on the west side, or left side looking at the 8. So you are central in the park Madison has dump stations and other perks. West Yellowstone has some great full hook-up parks. Many stay there and drive in each day. West entrance is in West Yellowstone.
Hotels in the park are outrageous hundreds of dollars a night. We use to stay there for less than a hundred in the '70's. Same for West Yellowstone hotels. We now just go for the day, we live 90 miles wast, or stay in our Class C in the park, or in Island Park, Idaho dry camp grounds for sixteen dollars a night and drive through the park.
Gas is no problem. Right now in Idaho Falls, Idaho its $1.80 a gallon at some locations. I would defiantly make reservations far ahead no mater where you stay. No mater were you go, depending on the month, well be loaded with people. We plan on going soon as the park is open just to see the new born buffalo they look like little orange drops.
One last thing. When driving in the park don't go buzzing around blind curves. There is a good chance someone has parked in the middle of the road looking at something, most likely and elk, and have no concept its a public road. Take it easy!