As others have stated its a good idea to open a vent in cooler weather and the AC is not being used. Ive been doing that for now for about 35 years and have very little window fogging. I know it sounds counter intuitive in that you are trying to heat your RV and opening a vent increases your heat load some but the way it was explained to me by many articles in Trailer Life (remember in the OLD DAYS before the internet Trailer Life was our pathway to enlightenment) that moisture build up was also going on in inside the walls of your RV and can cause damage.
Its "simple
" physics
1."humidity" always tries to balance and migrates from a high level (moist) to a lower level(drier)
2. Dew point . When the moisture in the air -a gas- reaches a surface that is at its dew point it changes in to water or if cold enough ice.
3. Temperature changes force migration as well. Walls are forced to "breathe" due to day/night expansion and contraction
Point to be made is if you have large amount of condensation on your windows, you are likely to have some inside your RV walls a well. Cracking one of your ceiling vents will help prevent this to a large degree. Our modern RVs with aluminum studs instead of wood, and foam insulation instead of fiberglass help prevent this damage but we still have wood materials on the outside (unless you have Azdel exterior walls) and inside.
I personally prefer to mitigate this as much as possible my turning the exhaust fan on the stove and shower and keeping a vent open at night as required.
Long story - but such is my understanding
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