Forgive my lack of computer drafting skills, the last time I used a CAD program it ran on MS-DOS 5.0. Anyway, the studs in the tear wall would be glued and nailed to the floor beam. The outer 1/4" ply comes down and covers the outside edge of the floor but still sits on the trailer frame. I like the idea of adding edge molding to help seal the gap and provide a more finished look. The camper shell bolts to the trailer with bolts inside the walls - I'd probably use a piece of steel on top of the floor assembly to spread the load - just a piece of sheet steel 3/4" wide by 2-3 inches long.
Basically, instead of building a floor platform then constructing the walls on top of this (as you would in house construction) the wooden frame of the body is a single unit more like what you'd see in a model airplane (or a home-built, actually) - it should give a strong structure and trim some weight. I'm still trying work out the dimensions for the floor.
That's the current idea. I'm trying to keep the wall loads in compression, maximize interior storage, and keep things relatively simple. It won't be as simple as solid ply walls, admittedly, but it should cut weight and provide space which can be easily insulated.