I thought about doing one like that but never could get it to work out like I wanted.
I saw a thread here yesterday that they did one on a utility trailer like that. Built an inner wall and the bottom plate of the wall was on top of the angle sides, and then they continued the outer wall down outside to the bottom. If I find that thread I'll post a link.
It would only work if you wanted the camper exactly as wide as the trailer you already have.
Just a thought.
Edit - Here's the thread.
http://tnttt.com/viewtopic.php?t=37652
If you did it like that and made it just as wide as the trailer and put a different front end on it - more flat and maybe a hinged/removable piece at the bottom where the rail is you could hinge it up and slide the whole trailer off the backa nd it would look exactly like any other trailer.