that goes to the engine block which has the negative leg of the battery laid down to the same bolt. The AC appliances are case grounded to the bonding system as well. There is also a galvanic isolator right behind the 30 amp AC plug in receptacle in the cockpit which I check periodically. FYI, This was copied from some artical I read some time ago. "There are two ways for a galvanic isolator to fail. Either the diodes are shorted, or they are blown open. You can test them with a digital volt meter that can read positive and negative voltages. At any time, with the voltmeter on the DC range, put it across the shore power side to the boat side of the isolator. There should always be some residual electrolytic voltage (unless you are hauled out) so the meter should read something less than one volt. If it always reads zero, the diodes are shorted out. If it reads greater than about 1.2 volts then the diodes are open circuit. Switch to AC volts and check again since if AC is flowing, the DC meter setting may not show any activity".