Yes ...you are right...sorry...but if the engine itself is connected to the ground bonding system then the angle irons don't need to be as they are connected to the engine.
Sorry for the incorrect information (the result is the same )...I;ll try to do better next time !!
opening all the DC breakers only disconnects the +ve feed, the negative is still connected to the appliances.
You may need to disconnect the negatives one at a time to find the leak.
SV Elyse SM437