I have similar door on my camper (maybe little taller in size). A mesh screen is attached to inner door grill. We never had issue with mozzie but midges can get through sometime. I taped that outer door vents at the bottom. It is source of dust getting in on off road tracks. I believe that vent is legal requirement when gas is installed in the camper. We don't have gas installed inside. Cold or warm weather, we are use to keep one window open while sleeping.
I am coming down the the Westcoast of WA and instead of taping the door lower vent as many campers and caravans were doing I fitted a piece of 12 mm foam between the inner and outer doors , found inch thick foam put too much strain on the door clips so just doubled it up across the top of the vent that with my positive pressure bilge pump and filter worked a treat