Black Mold on three roof vents after storing

Hi All,
I have a 2009 TM 3023 and each year after storage ( I keep my rv wrapped in a custom made breathable cover which is rain resistant but not rain proof), after I uncover it for the new season, I find black mold in all my roof vents and it's a pain in the rear to clean out. What I want to avoid is having to do this again. The first year I used a damprid bag which collects the moisture and hung it from the cabinet handle just inside the door. The second year, I purchased a good dehumidifier and kept it plugged in the entire storage time, but it was even worse. When the TM is collapsed, is it fairly airtight? Should I barely crack the roofvents to let air circulate? Should I try putting paper towels or fabric softener sheets on top of the mesh in the vent to help with moisture dispersal? should I put baking soda boxes around the inside of the rv to help absorb moisture?

If anyone has had this problem, I sure would appreciate your tips on how you were able to stop this from happening. I live in Seattle, WA so moisture here is more constant, which probably doesn't help my situation.
Thanks for looking and hope to hear some great solutions.
Jeff