Would you be trying to bring outside air in from the rear vent and then have it travel to the front and get pulled out from the exhaust fan at the roof? I am just trying to understand. If you ran this setup while driving, I would be worried about rear exhaust and smoke getting in from the rear floor level vent.
My bus is a Crown and it came with factory installed passive round roof vents to help get rid of humidity and create an air exchange process. I beefed these two up by putting in 12 volt quiet fans in the vents. They can be operated by a switch on the dash.
Round vent opening behind the driver. There are two of these near the front of the bus. They are normally covered in a fine mesh grill work so they look nice in the headliner. This picture shows the round hole with the mesh off.
This is the same vent hole with the headliner removed and the 12 volt fan installed. This was before we had the roof cavity spray foamed and the headliner put back up.
The vents are covered by a little hood on the roof to allow air flow while still keeping out the rain. Unfortunately, I can't find any pictures of the front headliner area in its finished state, with the mesh grill put back up and the leather headliner covering. My next trip to the bus.
Bottom line, the more venting you have the better.