Capacitive proxes?
"Capacitive Proximity Switches" are designed to attach to the outside of tanks and detect if fluid is present inside! I am pretty sure there are models that work on 12 VDC to control a small indicator light. I have not tried one as yet but will soon as installation is a matter of just sealing the end of the switch against outer surface of tank (no hole drilling). If anyone wants to experiment, these switches can be purchsed from most industrial electrical outlets, elctronics suply outlets and automation companies. As an after thought, they could also be used to control an LED in series with a resistor which would use less power.