Here is a video of the trailer in 20F with a lot of wind. I am venting pretty much as redneck described, about 1 square inch opening in the floor, with the roof level vent cracked open. With an electric heater, everything is cosy warm.
I am not going to try sleeping in it at 20F tonight, thanks.
I am still interested in a heat exchanger, because I would like to build a foam teardrop type camper. Using 2" insulation foam, build a 4'x4'x8' box on a trailer. I am interested if this type of foam trailer could use body heat to keep temperatures up to about 50F in the trailer, even in blizzard conditions.
An efficient heat exchanger could make this type of small shelter independent of a heat source.