many and varied.
I suggest using the Search function; using criteria like 'hitch mount'. You will find build threads, mostly for sedans. Look through some of the threads that sound interesting. Eventually you'll run across archetypal things like the Dryden truck
http://ecomodder.com/forum/showthread.php/most-aerodynamic-cargo-box-18000-3.html#post248933
Or aerohead's side views
You are right about an incomplete trailer tail. But an un-bulkheaded truncation can act as a 'box cavity' with beneficial effect.
Between a hitch mount and a full trailer, there is the single-wheel trailer.
BamZipPow's as an example.
Here is my own design for a panel truck I owned:
It would have a solid floor and inflated tubes or fiberglass rods to shape the top and sides. In your case a double hinged floor and pop-out rear bulkhead might work. One set of tail lights.