Hi, I don't know your coach specifics but many times the power wiring for the fan openings comes from the nearest ceiling light fixture. You might have a poor wiring connection (for either ground or +12) where the fan wiring splices into its power source. One way to check that general theory might be to kill the ceiling lights fuse and see if your 8 volts also goes to zero.....at least then you know roughly the source (and fusing for your fan). The other way is open up the closest ceiling light fixture (likely have to remove it from the ceiling too) and see if there is a splice there with wire colors similar to the vent opening. You can verify the wiring is for the vent opening and also check the splicing for vent feed wires (or perhaps a hard ground to chassis is used up there) is good.
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