Boring talk of weights & designs now. My concept of this trailer was that it would be empty inside (like a cargo trailer) with no cabinets, but with lots of anchor points on the walls for hanging and attaching things. I wanted a fold-down counter surface, hooks for hanging bags & clotheslines (for rainy days at the campsite), and brackets in the wall that would allow for knock-down shelves. The biggest sticking point, when I considered changing to a foamie build, was that I wouldn't have all those wall studs (2x2s)to attach things to.
My current plan is to build a minimal framework of 2x2s on the INSIDE of the foamie shell, so that I can have some of those anchor points. I don't mind having the wood show; it's like exposed rafters in a rustic cabin, but smaller.

Gluing smaller pieces of wood into the foam, traditional style, will not give me enough anchor points IMHO. We'll see how it goes.
Weight wise, my trailer frame is 260 lbs and the floor is 100 lbs. That's a total of 360 lbs so far. The rest of the materials- foam, 2x2s, canvas, a little 1/8 ply, 2 windows (10 lb each)- I can't imagine they will total much over 200 lbs. As GPW said, a cubic foot of foam weighs 2 lbs, and a cubic foot of ply weighs 57 lbs. So I feel optimistic about that.
After work tonight I'm hoping to skin the underside and flip it over, so that tomorrow I can waterproof the top of the floor. There will be pictures!