Disconnecting the chassis battery if it is sitting for 4 months and is not charged on shore power (some do and some don't) I think is a good idea. Keeping it charged is a better idea. Unhooking the coach batteries when on shore power is unnecessary. Keep an eye on the water. In any case it shouldn't damage the convertor unless wires were shorted. If it is sitting for 4 months and power is not available, disconnect all the batteries.
My 2 cents.
2016 Wildwood 28RLDS
2014 Tundra 1794 Platnum,"23" T-Bucket
2011 Harley Softail, 2010 Spyder RTS