Ian I agree with the LED answer (it was my assumption too).
I have also had problems with the water level meter. I traced it to a connection in the wiring to the the sender unit (in out case it was under the rear cabin bed. My guess is that it can be displaced (ie slightly disconnected) when servicing the engine (Or whenever the bed boards are moved) . In my case I squeezed them together and all was well. May be the same with the holding tank? To access that you would have to remove the wooden panel.
