Or use air.... Way back when we used to tent camp, we slept on a queen sized waveless waterbed mattress that we filled with air (the kind that has the baffles that are attached to the top and bottom). It was about 12 to 15 inches high when filled with air. Downside was that it was cold. But so are the air mattresses.
I have owned twin size Water beds, their nice... I don't think you would have a problem with one you may want to build a good frame to keep it from shifting while driving but sounds like a winning combo to me... I always enjoyed mine it was very comfortable. They use a minimal amount of water at least mine did. We use a Corda-Roy now http://www.cordaroys.com/shop/cart.php
"grease buddy" and all around nice guy