Battery alarm while moored.
Time to pull out the multimeter and check it. First, the obvious, what's the actual voltage of the batteries. Then measure the voltage at the meter and see what it's seeing. There are only a few possibilities, either the voltage was actually low, the wiring is flaky, the meter is on a point where there is a voltage drop (inadequate wiring/large load), or the meter is bad or programmed wrong. Another thought is electrical noise, are your sense leads too long and going past something "noisy" the fridge motor, a transmitter etc?
Ground is one of two wires, it's no more likely to be the black/yellow one than the red one..
1979 Contessa 26