Simply solution, I put a 5 gallon bucket over my trailer jack. It keeps water out of my jack (especially important in the winter when water gets in and freezes, then you blow a fuse trying to retract the jack) and UV light off the plastic. I hang the 7 pin cord from the bucket handle, so its covered by the bucket. I then attach my safety chains to the handle to keep it weighted down in the wind.
Kills several birds with one stone. But I agree with other advice; clean your pins and put di-electric grease to help keep water out and corrosion down.