We've been leaving the bathroom door cracked and running the fantastic fan on the lowest setting at night. On each bunk end we open one corner a couple of inches and you can feel the air get sucked through. We keep the furnace really low, only kick it on in the 50s and use the mattress pads and sleeping bags to keep warm. Not a problem this way. I've been tempted to try to put some sort of closable vent on the bathroom door so that in the heat we could leave the fan running and open up the top of the bathroom door (heat rises, get it out of the camper) and in the winter open a lower vent in the door so that the colder air is pulled out.
Maybe one day I'll get the guts to tinker with modding the bathroom door for that.
2016 Eagle 284BHBE
2015 Ram 2500 Laramie Longhorn 6.7L Cummings
Retired: 2014 X23B