I used a product in my Travel trailer when put in storage..It came in a small bucket like container with granules on the bottom and would collect moisture thus preventing mold and mildew. I cant remember off hand what it was called though...but it worked. I know Lowe's sells a product called Damp Rid but don't know how well it works.
Might be a product you could spray on for storage too...check RV cleaning supply sites etc. like
http://www.campingworld.com/