DC faults are hard to trace especially if there's more than one fault point. Let's assume if there's just one. Then first if you can, isolate all distribution ciruits (positive and negative, confirm that the main batteries are faultless, before tracing each indivual circuits. Alternatively, there're online earth fault detectors available in the market, where the detector injects and picks up a high frequency signal.
The DC earth fault locator is known as a 'Grouser'. I believe it injects an 11hz signal into the cable and is traced using a horseshoe shaped pick up unit.
