Air coming in cab area of class c when parked

Don’t forget that there is no door that shuts in the dash HVAC circuit when you shut off the engine. If there is an opening somewhere else that favours the prevailing wind, a roof vent, the exhaust fan over the cooktop, the fridge vent, you will absolutely feel a cold breeze coming in through the vents or floor outlets.

Try either moving the position to defrost and see if the draft changes, or setting it to recirculate, and see if it stops entirely.