Hey, Yooper. Welcome to the forum. Sorry, didn't see your introductory post until now. You've probably got your problem worked out by now, but if you haven't: here's my off-the-cuff/wall thoughts about that extra length:
It might allow for a longer, more tapered tail, thus better aerodynamics; aesthetically, however, that might look really strange, besides interfering with the requirements of a traditional stand-up kitchen.
On the other hand, if you keep the traditional TD aspect ratio on that frame, it would have to be excessively high to match the longer length. A "canned ham" profile might be a better match for that 12 foot frame.
Or maybe you could use 3 or 4 feet of the extra length, front or back of a trad-shaped TD body, for a porch of some sort. In front, it could be a platform for bikes and gear that wouldn't add greatly to the tongue weight. However, you'd lose the aerodynamic advantage of a close-coupled TD.
I've never built with foam, but it seems to let you create a round-cornered body much more easily than with traditional cabinetry techniques. DIYers make all-foam kayaks, dragon boats, and the like. So you could use the space ahead of the traditional TD volume to add a tapered nose, for especially good aerodynamics with the long trailer.
