If you're looking fo something not quite as large as the GMC, and want to stay rear engined, there is always the Ultra Van.  http://www.corvaircorsa.com/ultra.html

There were only about 380 of these made, and they used the Corvair 110 or 140 horse engine. This site states around 300 or so are still in existence, so the may be hard to find, and $$$. I'm sure these get a lot of attention in a camp ground!       Jeff