You have got to keep that overhead vent over the bunk cracked an inch or so, no matter how cold it is outside. I have had Bigfoot campers since 2002 and never a problem with condensation as long as overhead vents are cracked a bit. Have maxair covers on all vents. Over bunk vent stays open 24/7....Galley one gets opened first thing whenever stove gets turned on no matter the weather. I think your idea of insulating the front wall a bit more could not hurt either. A lot of moisture comes out of you breathing overnight right against that cooler wall. Probably some Reflectix would be good, and tape it in place so no air gets behind it.