The other thing is, if you can't find a leak, the mold might be growing due to high humidity in a closed-up trailer. In this case, it's good to keep air circulation available with open jalousie windows
if you have them, or if not, there's a product that will absorb moisture (I forget the name, maybe Damp-Rid? ...something like that).
Existing mold is best dealt with by cleaning/killing with bleach.