I'd second the Oregon coast, which is really beautiful. Beaches, lighthouses, many pull-offs to see great scenery, fun small towns, fresh seafood. We spent a week there in early June and really loved it.
Another relaxing option is just go up to the redwoods. Lots of campgrounds, natural beauty, easy walks. We stayed at Ancient Redwoods RV, right on the Avenue of the Giants. I would not recommend CA's coastal road (1) in an RV, even a small one, and I speak from experience. 101 is much more sane.
I wouldn't go to Yosemite, myself, because it is packed full of people, and even the free park shuttle buses are filled with lines to get on.
