There are a couple of things you can do. First, continue to leave the vent open so the trailer gets some air and the temperature inside/outside of the trailer can attempt to equalize. Second, if you have an electrical outlet nearby, you can install a small dehumidifier (you will need to have a drain tube and exit for the water to drain). Third you can place a temporary electrical fan while not in use or install a permanent exhaust fan on the vent closure in the roof. Lastly, you can purchase some chemical moisture absorbing packets from an RV dealer.