damp during winter storage

Hi my caravan is in storage, with is a secure unit but in the open, we covered the van in a breathable cover, and put dessacant around, however, for the very first time I have been intoday and it smelled of damp and some of the units had mold on them - I wiped everything down, locked opened windows, one of the units was slightly stained, any advice would be appreciated. I am assuming as we are in kent and this has just because of the severe weather conditions.