some simple tests
Yes, any boat could have a problem with the wiring harness. Your battery should go directly to the batt selector switch, with the shore charging system connected directly to the batteries. Try disconnecting the cables from the selector switch - this should isolate the batteries from the entire harness, unless you have a hard-wired circuit to the alternator. If your battery still discharges, it MAY be faulty - again, not unheard of even for a new battery. To further test, take it off the boat and charge it to full - if it loses charge then you know it's the battery - if not, you're back to testing your system.
