This is a very simple fan and it is easy to get to the switch. But first, is anything else on a 12 v. circuit not working? Perhaps the bathroom light, for ex.? If so, it is most likely the fuse and then you have to find the right one, check it to see if it burned out and replace it if so. You may not be able to tell if the fuse is shot—switch a few of the same amperage around to see if it makes a difference.
If everything else works, it may be the switch. If your fan is like ours, there are 2 screws on the part that pulls down. I seem to remember if you take those out, you can pull down the screen with the switch. Jump the switch and see if the fan goes on. If so, the switch needs to be replaced and you'll have to rummage around electronics stores to find the same one.
But also check the slide on connectors to the switch and motor. Pull each one off and clean them and put them back on, one by one (so you don't get them mixed up). They may have been loose, or corrosion on the contacts just needs cleaning.
If none of this works, it may be the motor is bad. Motors last a long time, but not all do. I think you'd have to look in RV stores for the same fan, or if you can find out the manufacturer of the fan, maybe order a motor from them.