You have not stated whether waste water has been drained into the tank from the kitchen sink, and or wash basin.
If water has been drained into the tank, then proceed as in post #2 above.
If the tank is believed to be empty, then this is a common problem. The waste tank typically has two electrodes near the top. When the waste tank is full the grey water makes a lower reistance path between the electrodes. This is detected by electronics in the control panel, and the warning initiated.
If the tank is left empty for some time, a bacterial film grows on the tank walls and simulates the effect of high waste water level. I find that after the tank has been partially filled, and the vehicle has been driven then the sloshing of the contents clears the contamination from the electrodes and the flashing alrm stops. I have seen an alternative solution of pouring a bottle of cheap coke down the sink, and the going for a drive. Why waste money on coke and fuel? Grey water often smells worse than the contents of the toilet cassette, so responsible disposal is necessary.
If on site I normally empty the waste water daily using a dedicated container, rather than using service points which were virtually unknown 30+ years ago. I note that your waste tap seems rather low, and unsurported. You may wish to take note of the attached modification to my PVC.