My transfer switch is just 1 relay.
It defaults to the generator side.
If it fails,.....doesn't switch to the shore power side when plugged in,
there is no ac power to the converter, (unless gen is running) and will of course run the batteries dead.
Why did it fail, ???? Same reason that all other electric things fail ?? They are not made perfectly.
Isn't this why you should always have a good digital readout volt meter in plain sight, showing 13.8 to 14+ volts as soon as you plug it the shore power. This would probably have saved your batteries