Welcome to the "Idiot lights don't work any longer" club (no offence intended, 60's term) . Ours became inaccurate within the first 6 months (4+ years ago) and died a little after that. The sensors in the tanks get coated and stop working. Actually they don't stop working, with the coating on them, they do not sense the water. As for the battery monitor, it is about as useful as the thank monitors.. so do not depend on that one either.
There is an outside chance that the connections on the back of the display/switches may have come off. You can remove the display and check the connections.
There are kits to change them to a "Capacitive" sensor$$$$ but we just empty them every few days.
Don
My Registry
RVing with SOLAR