Is it a galvanic isolator or just a monitor
Make sure that with the voltage conversion and changes to the boat that it is still in the ground circuit. If it is an isolator the ground lead from shore power
should be connected to it and nothing else, and the boat ground should be connected to its "output". If you remove the boat ground from it with shore power
disconnected, you should measure no voltage and greater than 100,000 ohms between the shore power ground and anything else ie. boat ground, shore power hot and/or neutral wires.
Having checked that, measure the voltage across it with the boat on shore power and normal loads. In particular any battery
chargers should be running. Measure the voltage for both DC and AC voltages.
If both voltages are exactly zero it has failed.
If the DC voltage is above about 1.2 volts it has failed.
DC voltages between these limits are normal but should be below 0.75 volts typically.
If the AC voltage is above about 0.5 volts you have some AC leakage on the boat. AC leakage does not directly cause electrolysis
but if it is higher than 0.5 volts it is decreasing the ability to block glavanic currents up to as much as 50%. Trouble shoot AC leakage by turning off everything one at a time until you see it drop, that is the device that has leakage and needs repair although some have built in leakage than can't be eliminated.
If you can't correct the AC leakage you can fit a Galvanic Capacitor. See the article on how they work at Galvanic Isolator Capacitor