Start looking for reservations in Yellowstone NOW! I would try for Fishing Bridge as it is central in the park. Yellowstone Grizzly RV Park in West Yellowstone and hit the Grizzly and Wolf Discovery Center a few blocks away, it is worth a stop.
There is also a few other campgrounds in the park so start researching. Enjoy!
EDIT: If you got a smart phone download the Just Ahead app
https://www.justahead.com/ for the Park. We used it and really enjoyed it. Had three grands and played it over the radio and we would kinda take a poll to see if anyone wanted to stop. There is a couple of other apps like it so you might look around. The $20 (?) cost was worth it for us first time visitors.