Ta Dah, I am going to respond even before the big game. It sounds like you don't think it is possible to build a light weight TD unless you go the foam route. Well, 10 years ago, I was cautioned to not even try to build a TD with only 1/4 inch plywood walls and roof. My feeling was, since I have been building cabinets for over 40 years, I could accomplish this, if stiffeners were added along the way to eliminate any flexing. I have even jacked up one corner of the rig, until a tire leaves the ground, and both doors can be opened without binding. I guess that tells us how rigid it is.
The curved roof was no problem, since it had cross members about every 10 inches, with 2 layers of 1/4 inch laun plywood and I can walk all over it. The side walls, rear of the doors, had interior cabinets and counter-tops butting up against them, there by eliminating any flexing. However, forward of the doors, I simply added a horizontal stiffiner, inside and out, and so far, after 10 years, I have had no problem what so ever. I glued and screwed everything and refinish the exterior every year with good old spar varnish. No insulation, no interior moisture problems and here in So Calif, I am warm as toast, with my li'l ol' elect heater that is about the size of a cigar box. This may not be the way to go in colder areas of the country where most probably insulate walls and roof.
The airconditioner in the tongue box, has been used exactly twice since it's installation, so it sits in the shop all of the time. I try to not go camping when the temps reach over 100 degrees, which happens every year where I live. If you look through my build pics, you can see how simple it was to build, and by the way, it's empty weight is only 450 lbs and is towed very nicely with my 2012 1.4 liter, turbo, Chevy Cruze. It gets up to 70 mph going up an on ramp to the freeway, which is something that my larger Toyota Camry couldn't do. I've hauled my woody across the nation 3 times, which means that it has seen it's share of mountains.
Do look over all of the info available on this forum, there are a whole lot of helpful folks here that can answer any question you might have. I suggest you find a nearby TD owner/user and pick his brain as well. Try to make it to some of the Gatherings, walk-ins are always encouraged, even if you have to sleep in the back seat of the car. Many have attended Gatherings with only a partially built trailer, me included. Good luck on your build, and by the way, Uncle Sam supported me for 2 years in Colo Springs while I was playing war games on the slopes of Pikes Peak. I loved the mountains, but hated the Army. Now when I go back, I can even afford to take the cog railway to the top of the mountain....whee.

Roly, the li'l ol' So Cal Woody guy