For a temporary option / solution this winter:
Cut out a piece of 3/4" to 1-1/2" thick, extruded polystyrene foam (corning pink or dow blue foam) the size of the cord box opening so it will be an interference fit when pressed into the door opening flange. Thickness determined by depth of cord box opening flange.
Drill a hole the size of the extension cord at a slight angle through the middle of the piece of foam using a sharp drill bit (maybe a brad point bit). Cut the foam in half with a knife vertically through the hole.
To aid in removing the foam pieces, stick a piece of fiber reinforced strapping tape, long enough to go vertically around the back and edge of each foam piece and then folded back on itself to make 3" tabs on each end.
Connect the extension cord, via the adapter to your existing cord and push the connection into your cord box. Sandwich the extension cord where it comes through the opening, with the two pieces of foam oriented vertically. Orient the pieces of foam so the hole slopes down to the outside. Gently press / pound the foam pieces into the cord box opening, with the strapping tape tabs led to the front.
Interference fit seals the opening and cord. Connection to the extension cord out of the weather. Slope of hole and vertical seam directs any potential water intrusion down, where the existing cord box flange directs it to the outside. Better insulation, one less cold opening into RV.
Pull on tape tabs to remove foam. If your fit is too tight, pulling on the tabs and extension cord at the same time, so the cord connection pulls against the back side of the foam, may allow you it remove the foam without breaking it, so you can reuse it in the future.