I installed a Maxxair in the bedroom and a Fantastic Fan in the bath, both with rain covers. They really pull in the air. I figured a complete air change in the trailer every few minutes. And, you can run them when it's raining with the covers over top, though they reduce flow slightly.
One other point to keep in mind. Make sure to keep all the screens in the trailer clean. The fans can't get the air out if none can come in. It's amazing to see how much air they move by the amount of dirt that accumulates on the outside of the screens. We use a soft brush on a vacuum to clean them. Have to do them two or three times a season to keep the efficiency up.
