The issue of RV roof venting is one of frustration for me because I like to run them constantly regardless of weather conditions. So when it came right down to it the only way to remedy a majority of my issues was to install a vent fan cover. I ended up installing them on both of the fans in my inside my class A motorhome. After it was all said and done it was a great addition and provided for much more flexibility when operating my fans in inclimate weather. I was reminded of this as there is a brand new video produced from RVgeeks and another from TrailerLifeTV regarding this very issue and I have posted both of them below. It gives a breif overview of this process and what to expect. As they show if you choose your RV vent cover wisely its a painless installation process.
Camco Aero-flo Roof RV Vent Cover used in the RVgeeks video:
Maxxair Translucent White Vent Cover (also offered in black) used in the TrailerLifeTV Video:
Additional resources for Installing RV Vent Covers: