Ok. So I have loaded pics. Also, I called my rv dealer and they didn't have a satisfactory answer. They suggested the same as you guys.
I did unplug the shore power and the light still came on. Don't know if that makes a difference but I assume that if it was for the tank heaters, they would not come on without shore power.
Also, I did not see a circuit breaker marked for tank heaters. Again, I don't know if they could be on a circuit with something else, but they should be pulling enough power that they would be on their own circuit.
I do have the original build for the trailer and it says it has tank heaters....unless it is fabricated.
Still at a loss and hope someone can help.