We just went through Yellowstone a couple of weeks ago. We stayed at West Yellowstone KOA, which I do not recommend. They really pack the coaches in tight. And the aisles are really tight for turning.
If you approach the park from West Yellowstone, you can do much of the park in 2 or 3 days. On one day do the Upper Loop, and the next day do the Lower Loop. You'll see most of the sights this way.
Know that Old Faithful has big crowds and you may have to wait an hour or more before either erupts again if you just misses it when you arrive.
Don't sweat it if you don't get it all in. There's a lot to see.
