Just some thoughts that popped into my head:
With no indication that you have any water at all, it sounds like you need to be looking at wires and connectors between the tank and the display board. My guess is a corroded connection somewhere or a rodent has chewed a wire somewhere while the RV was in storage.
On my coach, the tank sending wires all come together into a plug, then go up front to the display. Maybe that plug has come apart.
Good Luck, Be Safe and Above All, Don't Forget To Have Fun
2006 Fleetwood Discovery 35H, being pushed by a 2009 Honda CRV