Some of the original teardrops were built with 1/2" walls and no insulation. I camped at Christmas time in Virginia with my TTT which had 1/2" walls, and it was fine. Of course when I leaned on the walls they were cold, but the TTT with the ceramic heater was comfortable.
I have only plans to insulate the ceiling, not he walls. Walls will be 3/4 ply. When I camp it is never cold enough to warrent the extra time and expence in insulating the walls. Nor is it hot enough to need A/C. Ceiling will be 1/8 2x2 3/8, Alum. skin.