For large rigs > 30' you need a spot in Fishing bridge. It has been under construction for the last 3 years and is finally open for reservations for 2022. It filled up in the first few minutes of opening the reservations system for Fishing Bridge.
When we were in YS last year we toured each and every Campground looking for sites big enough for us (35' rig + 22' TV). There was nothing I felt comfortable with trying to get into. I am terrible for backing up. We stayed at KOA Mountainside. A little further out (10 miles) but nice.
The other KOA, Yellowstone Park / West Gate KOA Holiday, looks a bit tight. Not a lot of elbow room between sites. Grizzly is the best for this area.
https://www.nps.gov/yell/planyourvisit/campgrounds.htm