Packing under longer slat to ease sliding. http://deepredmotorhome.com/bed.php
expanding bed mechanism - Google Search
Slide out bed idea. Could be made lower, again would just rely on getting/making a futon style mattress. Air bed would work, however, would have to keep letting it down, pumping it up.
pull out bed campervan - Google Search
pull out slat bed - Google Search
DIY Camper Couch/Bed with storage.