Hello dandp and welcome to Air Forums.
You mentioned gfi - I guess you mean GFCI - but that's on the AC circuit. The AC circuit and DC circuit are separate. Your fan is 12V
DC so be sure you're checking the voltage on the DC setting on your meter.
As Iansk said, you could have a bad ground wire so be sure to check the + side of the DC circuit relative to a real ground rather than simply from one wire to the other wire.
Make sure your other 12V
DC items are working - maybe you have a problem with the entire 12V
DC circuit. Is your store/use switch working?