sorry, first (and second) posts here, apparently pic attachments do not work.

Remove the vent fan.

Roof venting is first & foremost about purging moisture to keep the roof deck from rotting. Installing a ridge vent makes the exit-point for the air at the highest possible location, guaranteeing that the entire roof deck gets (convective) air flow from stack-effect pressure differences. A ridge venting scheme also needs corresponding soffit vents, to guarantee that the lower part of the roof deck gets flow as well. A roof fan at the midpoint doesn't guarantee where the outdoor air is coming from, only where it's leaving, and it significantly depressurizes the entire space. In a garage a large fraction of the entering air would be leakage around the garage door, and most of the air flow would be short-circuited direct to the van (or ridge vent, if there are no soffit vents).

Is the garage currently (or will it be) insulated ?

Where is this house located? (By IECC climate zone: https://basc.pnnl.gov/sites/default/files/styles/large/public/images/IECCmap_Revised.jpg?itok=2R5NbX29 )

A safety issue: NO FAN unless it has an explosion proof motor.
If you happen to have a gasoline spill, or other flammable liquid like paint/lacquer thinner, inside the garage an exhaust fan motor starting might create a spark in the motor which can ignite the fumes. BOOM!