Air coming in cab area of class c when parked

On my Chevy Kodiak (cab is Express van with wide fenders) there were vents in driver and passenger doors. When I put new speakers and sound deadening in I closed off the vents. Made a big difference in how much air came inside cab area. There are enough leaks elsewhere in motorhome to make up for the loss of this air flow.
Before shutting engine off at a campsite would turn vent selector switch to recirculate as others have stated.