Shore cord to trailer is a Marinco connector .. They are designed to be very water resistant.
If you want to "Extend" I do not weather proof but I can weather resist. you need two thins
one is a modified plastic tool box I cut notches in the ends to pass the cords and drilled holes to drain any water that enters the tool box. These are cheap boxes.. Set it atop any riser (Milk crate, for example)( and sand bag it agains falling off and you have a very weather resistant connection,.
Or set the milk crate on it's side. Tie trhe box to the top side, and set weight (Couple gallons of water? inside crate as ballast.
