If all your fuses are present and good, check the voltage to ground on each wire.
If one is 12V and the other is ground, then you can install a 12V outlet or other 12V item like a switched night light for example.
If one is 12V and the other is neither 12V or ground, then it operates an appliance of some kind. If that is your case, it is possible a switch located there once controlled your black and gray waste tank heaters. It seems most people are perplexed over their "mystery" switch, which most often is the tank heater switch. Being as old as your trailer is, it is also possible a previous owner removed the tank heaters because with age heaters can give-way and hang down. So adding a switch there would do nothing for you.
A built in switched LED night light might be a nice means to utilize the hole assumed you have a hot 12V wire there. You might have to fish in a ground wire from a nearby ground to complete it.
