If your fridge is in a slideout there is no roof vent.
The fan is there to keep a draft moving over the fridge coils. It is a thermally controlled motor that has a thermal switch attached to the hot pipe. Most of them do not run all the time, just when the ambient temp is high or the fridge is working hard to keep cool.
2011 351RLTS Eagle, Mor/Ryde suspension & pin box
06 F350 Lariat PSD, SRW, LB, CC, EGR delete kit, 16K DrawTite hitch, Timbrens, TST TPMS
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