I gave this considerable thought a few years ago. I initially thought about modifying an egg to have a back door, but decided that wasn't a good idea for several reasons: "Floor of egg not built to support that sort of weight" was the big one - but there were others. So, I eventually decided to approach the problem from the opposite direction. I found a nice cargo trailer (It's a HiPoint trailer, fiberglass structure with an internal steel frame supporting it) and built it to be a camper. That way I could place everything exactly where I wanted it, get everything out of the way of the bike (and handlebars), have the side beds fold up etc etc. It's not as built-out as a typical camper, but it does OK.
Course - pretty much as soon as I got started on that the twin girls came along, plans changed and I really haven't had a chance to use it. Anybody in the market for a miniature toy hauler?