As mentioned, first look at the fuses. There's a couple behind the left
rearmost pillar (in a westy, you'll have to remove the shelving in the
cabinet to gain access). If that isn't the issue, get out a volt
ohmeter and check for power along the wiring. First at the switch that
controls the fan speed then at the connector in the left rearmost
panel.

Last year, I discovered that the connections on my connector had
oxidized enough to cause the fan not to run (and was on its way to
trying to melt the connector.) A cleaning of those contacts was all I
needed to do...but I had to go through the labor of checking everything
along the way first.