Being a pre-owned RV Trailer this switch could be anything...
On your pushing the switch in question be sure to have someone outside your trailer looking for something to come ON and OFF.
I ran into this situation on my 5th wheel trailer with a mystery switch... My switch was a standard wall mount light type switch... I turned it ON and OFF and could not determine what it was controlling. Later on I was doing it again and my wife just happened to be outside and she came in asking did I know the two SCARE lights mounted up high close to the rear on my 5th wheel trailer was coming ON and OFF...
Google Imagine
To me this looks like a previous owner add-on for something which may be hard to trace wiring... Perhaps you could ask the previous owner if it was a private purchase....
Hopefully someone one here with same type of trailer will respond...
Roy Ken