I can't vouch for this personally, as I am still in the process of finding an Airstream to call my very own
, but based on many online recommendations (and the fact that it was on sale), I purchased The Holding Tank Cyclone, made by Camco. It's designed to pull waste gases out of the trailer using the cyclonic effect; you probably are aware of these gizmos.
But it occurs to me that adding the cyclone to the vent opening would also
offer additional protection from the elements for the vent opening itself.
Just a thought. Good luck!