We filled in August -Mid September last year at Hunter Peak Campground just up from Crandall on the Chief Joesph Hwy along the Clarks Fork of the Yellowstone. This year we are going to be at Crazy Creek Campground just about 10 miles(?) east of Cooke City, Mont. in Wyoming. We'll open the campground on May 21st. We have access to the Clarks Fork of the Yellowstone and its tribs. along with the Beartooth Plateau High Lakes. Soda Butte, the Lamar, and Slough Cr. are about 1/2 hour depending on Road Work.
At the risk of being called a liar, The fishing is fantastic, Outside Yellowstone is really as good or better than in it. Examples, My wife and I have caught 50+ grayling (10-14 inches) on dries in one afternoon. I got into a spinner fall on the Clarks Fork of the Y one evening, 35 fish in 2 hours. I can easily can pick up a dozen Brookies (10-12in) for Dinner, they're invasive up there-(reverse situation of GSMNP).