There was a guy in the Lewiston, ID area that had cut the side out of an enclosed cargo / snowmobile trailer and retrofitted it with the drop down side of tent trailer with similar bracing the tent trailer uses when down. I wish I could find it, so I could share. A friend of mine found it on craigslist.
Rigid exterior when folded up for road use, and nice queen bed set for the tent side when set up for camp.