Did a bit of insulation on the trailer the other day. Screwed a big piece of styrofoam right over the "front" door which is right next to the bed. I am gonna put a rope through and across it, so in an emergency, I can rip a hole in the foam, open the door through the opening, and then just push the foam out the door. There are two ceiling vents, about 14" square, cut pieces and pressed them up in those. They are single layer plastic and ZERO heat retention. Ditto the big skylight in the so called shower. I will screw some foam up against that as well. Two of the biggest losses are the big 48X24 windows, 1 on each side. The windows all open and only single pane. It will turn the trailer into a dark dungeon.
The main (rear) door is a leaky SOB and not too much I can do, there.