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
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.
, 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.