By the way a company I worked for had an old Spartan Manor on the property with a swamp cooler, worked great and no problems with mold. Yes you have to keep windows open, that is the first rule of any swamp cooler on any structure, trailer or not. The humidity a swamp cooler puts in the air is a lot less than the breeze you will get in some of the more humid locations and it will not build up unless re-circulated or trapped which is not how a swamp cooler works. It's a cool breeze flowing thru your trailer just like the one that comes thru your windows, it doesn't build up the humidity level. Like I said in my first post if you haven't lived with the concept in an arid region you just don't get it. Even some folks who have, but didn't understand the concept of air flow don't get it. If you don't have a complete changeover of air from outside every ten minutes or so you don't have enough windows open, and you won't get the benefit, and yes, you will build humidity.