Well, you spooked me, so I went out and checked our 11 yr old 5er that has been sitting all summer - and it's fine. Even in the 70+ degree dewpoints we've been treated to this week (yuck). But I agree with others - somewhere, you've got a wet spot that is creating the perfect indoor environment to grow mold?
My pontoon boat gets mildew due to a non-breathable cover - I use a product called X-14 to clean it and kill the mold - but keep in mind that most mold-killers contain bleach. Be very careful where you spray it!