To be honest, I'm not sure about "water proof vs. resistent". What I do know is that you want one with fabric that "breathes", i.e. one made for the purpose. Moisture will form underneath the cover, so you want one that will allow that to dry. When looking under mine after a rain, the sides have a very fine mist on them, which dries quickly. Most covers designed for rvs will do this. Hope that helps.
Jayco Designer 30 RKS Medallion pkg, Trail Air pin
'05 F350 6.0 PSD CC 4x4 DRW LB B&W Companion, Edge Insight
“Certainly, travel is more than the seeing of sights; it is a change that goes on, deep and permanent, in the ideas of living." Miriam Beard