I also have an inspection hatch thingie in the bottom of my power cord compartment.
I took a funnel and cut a slot in it to go over the power cord (and duct-taping the cut), so it fits into the round inspection hatch so that no critters can get into the compartment.
If you wanted to be fancy, you could probably buy a spare inspection hatch (they are common boating items) and mount a receptacle of sorts onto it.
On the other hand, should mice get into the compartment it's sealed from the rest of the coach, so they probably won't cause many problems. And you could always put D-Con or Peppermint Oil into the compartment.
We used Peppermint Oil last season with great success. Mice don't like the smell, so they stay away. We put 4 glass cups in the inside of the RV with a cotton ball in each one - soaked in Peppermint Oil. It made the RV inside stink though, but the smell dissipated within a week of opening it up when we commissioned it last spring.
If you use Peppermint Oil, make sure you use glass containers as it will eat through plastic cups.
This is what we used:
It does not kill the mice (for those of you concerned about that). They just don't like the smell and they stay away. The stuff is called "Mouse Away", not "Mouse Be Dead".