quick question
did you turn off the built in power converter ?
you should unplug it or turn off the circuit breaker that powers that circuit
it might not be the source of the 'error' code, but it would create an endless power loop and drain the batteries
inverter using battery > converter using inverter power to try and charge that battery > which never gets charged because inverter is on
no such thing as free perpetual power
a leaky power cap or diode in the converter MIGHT give the neutral ground error
just something to check an eliminate