Don't know how often you wash it. But the grew mildew is dust that turned. IF it is kept clean. It won't mildew as bad. I would suggest washing it every few weeks, and waxing it 2 time a year. You see that a lot when RVs are put on a perminate site.
Terry & Shay
Coachman Apex 288BH.
2013 F150 XLT Off Road
5.0, 3.73
Lazy Campers