Fridge propane being blown out while in transit is due to 'body design' creating turbulence along the side of rig.
Usually hear about this with truck campers and Class C due to 'camper' being wider than cab.
Sidewall of travel trailers and 5th wheels is relatively smooth and air flows down sidewall without swirling.
Running with fridge on is perfectly safe. They are designed to operate while in transit....with features to shutdown safely.
Normal hwy speed isn't the issue.......windy conditions are and not much you can do to control winds
Blocking off lower vent will just result in less cooling due to obstructed air flow (draft effect).
You could install a sheet metal shield like the Dometic Recall Kit around burner area.
