That is an unusual location for a factory installed light. It makes sense to have one there because of its proximity to hookups and utilities. I Google searched your trailer, and found some photos online of a couple for sale. Neither had that light. Is it possible that this light was added by the previous owner to assist him when hooking up in the dark? Could it be just a tap light where you push on the cover to turn it on/off? Is there a toggle switch on the light itself? Is it just screwed or adhered to the side of the trailer and runs off batteries? Can you remove the housing to see if there are batteries inside?