We have no heat but an electric blanket or our own bodies. When we camp on a cold night we use the electric blanket. When there is no electric we use a down sleeping bag over us. Both windows and the top vent stay open. Also I have a simple hook mechanism for each door to hold it open a crack. We have never had a problem with condensation. (Yet!)
I see two areas where we might be lucky. Our camper is ventilated severely compared to most. Our camper is completely open inside with no cabinets or kitchen so has a larger interior volume than the typical 4x8 tear.
Tt