Mount Rainier is an excellent campervan destination, but the park has very limited camping so we defiantly recommend planning ahead and making reservations well in advanced. There is more campgrounds outside of the park if you don't get a spot in the park before your trip. Our favorite campground is the Ohanapecosh campground since the trees are just amazing.
There are numerable trails in the park to explore. The trails on the west end of the park do tend to be more crowded so if you want a little more remote experience then travel to the west side of the park.
Campervans available for rent in Boise, ID and Salt Lake City, UT.