Knowing what year and model it's in might be helpful in answering your question.
From the location you described, it might be the switch that shuts off the converter (or just disconnects it from the batteries?) when plugged into shore power. That was a feature of some of the longer trailers in the 70s and 80s (and possibly longer) to help prevent overcharging the batteries. I don't remember seeing one with lights, but I've never had one in a trailer I owned so it's just a guess. From the look of the switch plate, it may be a Previous Owner Special Project.
Zero Gravitas — 2017 Flying Cloud 26U | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill