Yes, respiration during the night will cause humidity, especially on the windows behind the closed curtains. It sounds gross, but a person will loose up to 1 quart of moisture, during sleep, over night.
Opening the vent (1/2 inch or so) won't loose you much heat and might be more comfortable along with helping relieve the morning moisture build up.
I would suggest opening the vent open just enough that the lid still laps over the frame. This way if it rains during the night, it shouldn't rain in, that is, unless it's a driving rain.
We've never had frost on the walls, but that's just our experience.
Running the fan shouldn't be necessary unless it gets stuffy and you want some fresh air. The CO detector is hard wired in (no worries about bad batteries) and should be warning enough if there is a problem there.
Our CO detector actually went off one night. All we can figure out is it was from our breathing and the Aframe being buttoned up tight (can you say breath mint?).
When cooking we open the vent and run the fantastic fan to vent out that moisture.
Closing up the camper for storage, with excessive moisture in it, can cause the mold issues others have had problems with. That's one reason I like to open up our camper when we get home and air it out, especially important after camping in the rain. Also, I leave the bed unmade between trips, so the mattress will air out.
Deb & Gary
2007 Ford E150 cargo van, 2012 Rockwood Premier A126
Everybody needs beauty as well as bread, places to play in and pray in, where Nature may heal and cheer and give strength to body and soul alike._ John Muir