As Donna points out, there's more to it than it seems at first -- Some of the inner parts are needed to support the shell -- Also, to mount the inner parts, like the rear benches, for example, there is factory glassed-in wood for support and to receive the screws -- Likewise, the supports for the front bunks.
I can understand wanting to do this, but you'd be reinventing the wheel in a lot of areas. You might have better luck buying a non-fiberglass cargo trailer and outfitting it as a camper because you would be starting with a complete stand-alone trailer and just adding accessories, hatches and
windows to it.
http://www.wellscargo.com/wells/cargo-trailers.html
Friends of mine in RV park in Florida had a long one that they had modified the front half as an RV, in which they lived, and the rear carried his race car.
Burro, when they were in business, used to sell kits, but all they really were was a factory rig that hadn't been assembled, not a blank slate.
PS It's actually MORE work for a manf to make something non-standard because its difficult to know where to stop...
My Dad knew a taxi company that wanted to put Chrysler-Nissan diesel engines in their new taxis and was going to order a bunch of them without the gas engines -- They were told that it would cost them a lot more for the non-standard order -- They finally ordered the cabs complete and let the company installing the diesel engines keep the new 318 V8 engines as partial payment.