"A picture (on your part) is worth a 1000 words..."
I think that's a good design, although I do believe I'd err on the side of caution and stick with 3/16".
Any plating you can put at the joints for added strength, in my opinion, would just be a plus.
No cross memebers for goodies on the tongue down the road? What is the finished weight of the trailer..loaded and unloaded?
I really like the truck body trailers...lots of potential.