We were in Yellowstone last September. Campgrounds were totally packed, crowds were everywhere. Reserving an electrical site is ridiculously expensive, and the other sites are snapped up very early every day, usually by those wanting to upgrade to a better spot.
It's high country so be prepared for anything. We left with snow coming that day, but it seemed like a twenty mile string of people on the road heading in just the same.
We really liked Woodhead Park at Brownlee Dam, Idaho. The campsite is a bit deceiving, first glance might not reveal all the sites at the other end of the park, many with spectacular views.