Originally Posted by padlin
So is the purpose of a heater to keep it dry?
A heater doesn't remove any moisture at all; however, warm air can hold more water, so keeping the air adjacent to surfaces a little warmer means it holds a little more water so less water condenses out on those surfaces (like dew). A warmer trailer has less damp surfaces.
Edit: Tim posted while I was composing. I completely agree with his excellent explanation.