Not sure what you mean, please clarify as to what the fan doesn't do. If the fan doesn't blow and the doors don't flap then I think you are looking for a power issue. Get the owners and possibly service manuals and look for fuses and relays. They should be numbered. Remove fuses and test with a meter. For relays swap them for another identical one in the box and see what happens.
If the fan runs but the doors don't flap then you have a vacuum problem or a power to the relays problem. For gassers these are under the hood and beneath the dash and are either black rubber or thinner colored hard plastic. Start at one end and look for cracks or loose hoses.
There is more but check these obvious things first.
33' 2008 National Tropical on a Freightliner chassis.RVM#189