Although we don't live on the "Wet Coast" we have never had to use a de-humidifier in our non-insulated and carpet walled fiberglass trailer. We have used it since '94 and good ventilation has kept it pretty well moisture free. Even in rainy, damp, or cold weather there's always a window open, if only a crack and with the awning
set up we can open a the window wide and sometimes even the door. There's also a small computer fan in the kitchen window that helps to create a cross flow. If you close things up too tight then you'll find out how much moisture humans can add to the inviroment.