The Billings Metro KOA (3087 Garden Ave., 406/252-3104 or 800/562-8546) has 115 RV sites, 60 tent-camping sites, and a pool, playground, and minigolf course, all right next to the Yellowstone. The KOA is off exit 450.
The Billings KOA has the honor of being the first KOA in North America.