Things inside a TT that produce water vapor.
Exhaling - the human body expels water vapor while breathing. You can produce a quart or more of water vapor just while you sleep. So figure one person can produce 3 or more quarts per day.
Cooking - moisture boiled off while cooking either in the oven or the burners on top of the stove. Propane combustion also gives off water vapor as a byproduct of combustion. All microwaves are vented at the top rear of the cabinet. Although they are usually mounted over the stove hood in trailers they are never vented by the stove exhaust fan. The microwave exhaust usually winds up coming out of the grill at the front top of the cabinet enclosure the microwave sits in.
Propane heaters - Mr Buddy, Wave 3, etc all will add to the humidity level due to combustion.
Showering - hot water and cool air means more humidity.
I normally kept both roof hatches cracked about an inch. When it was too breezy or cold and I had to close the front hatch I ran a 25 pint per day whirlpool humidifier set at 40 percent humidity. That was suitable for me and one Chihuahua in my 28-1/2 footer.
