You answered your own question - Shore Power is the answer. There are marine chargers that are made that will monitor and maintain different battery banks. I have one in my Houseboat and one in my Motorhome, the boat is for three banks, one battery is a large golf cart battery for every day 12 volt usage, one bank for engine starting, one for the generator. A computer in the charger monitors the batteries and controls the charging, from trickle to a heavier charge. The boat generator runs twice a a day to bring up the golf battery while away from shore power, hence a heavier charge, at the dock with shore power it may only need a trickle, These chargers are not all that expensive, the one in my motor home paid for itself in the batteries it saved, it is only a two banker for engine and coach, paid less then $100 for it on Ebay.
When I answered, I thought it was in reply to a question to keeping batteries up, the inverter in the unit can ruin batteries if plugged in all winter as you know. But my answer was not posted where I thought it would be. Regardless, it is a good alternative, marine chargers will float, trickle or charge depending on the battery per bank.