Mystery "plug".... in my 2007 Aspect 26A

[S.Rodimus]

I'm only guessing that this belongs in the Electrical section, because the "plug" I'm asking about is right next to a normal 120-V AC outlet -- but of course I could be wrong....

Does anyone know what this is? It's located on the back wall of my slideout. I have no "key" or anything else that seems to fit into it, and it looks to be too big to be something like a 12-V DC "cigarette lighter" type outlet. There's nothing about it in my Operator Manual.

Sorry if this is a dumb question. Thank you!