What all are you storing in the trailer? If its just a car and I wanted to keep it protected I think I'd do this ...

Buy one of those large storage bags and drive the car in. Wait for a cold day when the humidity is low - middle of winter works - and then use a shop vac to suck the bag tight to the car. Seal the bag.

Now the only moisture to form on the car itself will be the minimal amount in the bag. Moisture in the outside air will condense on the outside of the bag, not on the car.

Just a thought.