I'll second all those recommending Jedediah Smith State Park. I would definitely make sure that you get reservations during the summer months. Also be sure to check the max RV size. We have many fond memories of camping there with our kids. The Smith River there is pretty shallow and gentle and the fog from the coast seems to want to stop at the park entrance. We are hoping we may be able to go back there this summer with our grand kids this time.
Jim