Tim
Let's narrow this down a bit.
After you've done all your cleaning and insulating and with the battery pack disconnected at BOTH ends, do you still get a voltage reading from any or all battery terminal(s) to chassis ground ??
If you do, there still is a dirty battery or one of your cables has a crack in the insulation allowing moisture in and causing your fault. Make sure that there is no direct connection of pack negative to chassis ground anywhere in the system.
If there is no voltage reading, the next step would be to connect the charger to the pack and only the pack. Nothing else, and activate the charger. If the GFCI still trips, I would suspect a fault in your wiring from the charger to the batteries, or your 120 v ac line. Could be that the hot and neutral wires are reversed.
If by chance that the charger works fine with both ends of the pack disconnected from everything else, and you reconnect one end of the pack back to the car wiring and it faults out again, then the fault is in the car wiring, the motor has carbon tracking, the controller has a bad or going bad capacitor, some tracking around the main contactor, the dc/dc converter is causing a fault, your electric heating element or heater relay has a fault, the circuit breaker and or fuse or shunt has tracking around it. All the small wiring that carry pack voltage are suspect. Wires running to the voltmeter and ammeter are usually small and can get damaged and cause this kind of fault.
If there are flooded lead acid batteries under the hood of the car, every device also under the hood is suspect, because the fumes (mist) from charging the batteries gets on everything.
John Emde