Air coming in cab area of class c when parked

JaxDad wrote:
....or setting it to recirculate, and see if it stops entirely.

That would be my first advise.

With the engine running, set your heater control to "recirculate" to close off fresh air from the outside.

But I like to leave it open to allow air inside when running an exhaust fan in the house.

We drape a comforter across the two front seats and seal up around them with it to provide a cold air barrier.