There's a couple things you could do, but I'd first make sure that I was assembling all of the required parts that keep moisture and crud out, then I'd start the assembly procedure by filling the connector with dielectric grease.
Don't be shy, the grease is non-conductive, and non flammable. Fill all the holes before you insert the wires, and just let the stuff squish right out of there. give everything a liberal coating before you put the cover on, then fill the holes where the trailer connector goes.
Yup, it's messy, and you'll want to pressure wash the outside every now and then to keep the grit out, but it will keep out the water and corrosive gunk.