When we first got our Hi-Lo, we thought our refrigerator fan didn't work. Others here on HTF explained to us that this fan has a thermostat, and even when the switch is turned on, the fan won't run unless the thermostat kicks it in. Happily we discovered that our fan worked fine.
You may already know this, but it doesn't hurt to make sure. Since you have already checked the fuses, I tried to find a good wiring diagram for the fan circuit, so that you could test the circuit. The best one I found, and it’s not very good, is from Dometic. It's here in our library, here's the link...
I found this to be true also.
I thought our vent fan was bad and started looking around for a new one when I figured out the fan only runs when its needed. I'm glad I didn't buy a new one, they don't exactly give them away.
In addition the the "only runs when needed" comments above, did you check the inline fuse that is only accessible from the outside? You have to take off one of the outside vents to get to it.