Island Park, ID can be a good base camp for at least some of Yellowstone (about 15-25 miles from the west gate of the park, depending where in Island Park you land). From there you can also spend 1/2 day or so at Mesa Falls (beautiful)! and even head up to Virginia City MT - old mining town - or Hebgen Lake area (Quake Lake, etc.) Or south from there to the Tetons, via either ID 33 or (better route for RVs) US 26 - longer drive but pretty. Definitely you'd need reservations in the Tetons unless you can dry camp.
Craters of the Moon NP is a neat little park, and very different. You can camp there (hot & you have to dry camp) or base camp in Arco ID, about 20 miles east of the park; on US 20.
