Caravan door vent leaks

In recent wet weather, we've noticed a fair amount of water seems to get through the standard vent in a Camec (?) caravan door, particularly when the wind is blowing into the door side of the caravan. The vanes of the vent are directionally opposed between the external and internal sides of the vent to exclude water coming through while driving - this seems to work OK. However, while stationary in rainy conditions with the wind from the 'right' direction, our mat just inside the door gets quite wet. We're sure we are not the first to experience this problem and we're wondering if there is a neat and simple solution, apart from taping a waterproof membrane over the external vent when rain is expected? We would appreciate any advice and a reference if this issue has been dealt with on the forum previously. Mike & Helen.