[twindman]

Fishing Bridge campground is STILL not open until maybe Sept. ANything in the park needs a reservation and probably not available at this late date. By using a genny you can probably stay at several parks inside the park if you can book it. Some things depend on how big your rig is. I am not sure you can squeeze in with anything over 35 feet, but I may be off.

As someone else said, W. Yellowstone has many campgrounds, but my guess is they are booked already too. Grizzly RV park is really nice and about 3 blocks to entrance, but very expensive.
And not mentioned here, is that with the Covid shutdown rv's were sold out last year and those people are all going camping now. Good luck.
p.s. I got to Yellowstone about every 4-5 years since I have family 20 miles east of Cody, so am always in the neighborhood.