Are you coming down through Colorado? I70 west is an awesome drive--you'll hit all the major ski towns--Frisco, Vail, Glenwood Springs, Grand Junction--which are still pretty cool in the summer--lots of mountain biking etc.
I'll second Crater Lake. Also, I haven't seen it mentioned yet, so also the Grand Tetons right outside of Yellowstone.
As far as Yellowstone goes, take as much time there as you can spare. You can't see it all in a day or two. I had no idea how big it was when I went. I ended up staying 7 days (which is the limit for RV camping in the high season I think) and would have liked to stay a couple more. There are about a dozen different campgrounds and they all have something to offer.
I had one of these guys use the front bumper of my RV for a scratching post (while I was in it) while trying to cross the wooden bridge. :sorry:
I freaked out and rolled the windows up and locked the doors. As if that was going to help.......hone: