Wouldn't you get condensation if you heat the trailer all night and then open the doors to single digit temps at load in? Seems to me you would want to start an equipment "cool down" process before you load the trailer.
There might be a little bit of condensation as the warm air in the trailer dumps its moisture because its being cooled down, but it would be minor. In order to have condensation, first you need to have air that's loaded with moisture (being from the south, you'd be familiar with humidity?
). Then you need to cool that air, so that it can't hold as much moisture, and the water vapour turns to liquid water. That mostly happens to the air that's right next to the cold stuff you just took out of the trailer, otherwise it would start raining in the room. It doesn't happen in the other direction (warm gear into a cold environment), because the cold air contains very little moisture.
It's not usually even a problem, because there is so little humidity here at this time of year. The exception is rooms full of people. At the end of a gig once, I brought the cases for the gear into the bar after they'd been in the truck all night, and a layer of frost formed on the metal edging almost immediately. It would not have happened if the room hadn't been full of inebriated waterbags, dancing up a storm all night.
GTD