A Fan-Tastic Fan is a fan that installs in your 14 x 14 vent in the ceiling (should be a vent in the bathroom, and you may have others). Here's their website: FanTastic Vent - Home Page
. Camping World and most RV places sell them. They are typically 3 speeds. Fancy ones have remote controls and I think some are reversible and some have thermostats. In spring and fall, they're all you need. They are also excellent for pulling the moist air out of your bathroom.