you could adapt a flip-pac camper and get something like this
http://bajataco.com/vikingvince/Vince_FlipPac/index.htm
http://www.flippac.com/
It is an easy way to get a lot of room and a comfortable bed and you build the interiror to suit your needs.
No association with flip-pac by the way, just an idea.
Regards
Fernando