I assume you pushed the button to turn on the meter, and only the "empty" light came on. This is the opposite of the usual problem with this type of level gauge, in which crud sticks to the sensor electrodes and shows wastewater that isn't there. The only ways to have missing indications areThe tank level monitor for the gray tank does not work. The tank overflowed into the kitchen sink and the monitor said it was still empty.
- faulty LEDs in the monitor panel (extraordinary unlikely that all would be bad)
- bad wiring connections from the sensor electrodes to the monitor (unlikely that all levels would be bad)
- bad wiring connection to the reference electrode at the bottom of the tank.
I'm surprised that the grey wastewater backed up all the way to the sink level instead of coming out in the shower - that shower drain must seal well when closed.
As reported and discussed in How does plumbing vent system work?, the 19 Foot and 5.0 (original single axle) have a hidden air intake vent pipe, which could overflow inside a cabinet if the gray water level gets this high. I don't know if other models have the same configuration, but if the 5.0TA does (ask Reace) I would be opening up the cabinet with the vent to see if all is okay.