I went camping with a GF a few weeks back and made the following observation. (I'm not surprised by the outcome but it was interesting to actually experience the difference.)
I have a
UHaul, double hull, smooth interior (which I love) and my GF has a
scamp with ratfur interior. During the late evening and through the night, my trailer got dampish feeling and chilly, so I had to run my cube heater. My GF, on the other hand, never ran her heater. I was at her trailer for some wine and girl talk during the evening and the temp was just fine. She said in the morning that she never even turned on the heater. Now she is the 'cold' one of us two, and if anyone was goning to get chilled, it would have been her.
So as it has been pointed out before, a lightly insulated 'liner' material on the inside of a fg trailers really works. If there was a way to get my
UHaul insulated, I'd jump at it!