I pack in some foam around the cable and close the smaller slide door tight on the foam which pushes it around the shore power cable. All snaps down in place with the foam installed. My problem is always ants and bugs more then mice getting in...
photo from google - not mine
My OFF-ROAD POPUP had many small holes under the floor where wires come through from the top. Took awhile to find and stop-up all of those... I finally ripped out all of the wiring under the trailer and installed 3/4-inch PVC flex lines and PVC electrical junction boxes with a large hole in the center aligned to the holes that came thru the floors. This sealed up all of the large holes under the floor area. Don't have that problem anymore now...
You can see how this looks in this floorplan drawing of my off-road popup shown in BLUE. No more holes in the bottom anymore. I also have a way to fish cables from point A to point B as well from the top.
This will hard to do on your trailer as it probably has an under belly membrane installed.
When I start on my next battery upgrade I will replace the pull out cord for a screw on shore power connection cord. This will fit into my alternate ways of providing power for my OFF-ROAD POPUP. The power connection of the right side of this photo comes from my 1500WATT PSW on-board Inverter.
Roy Ken