Batteries lose power just sitting. If they are not kept charged they will sulphate and have a short life. You are better off to have a charger and use 12 volts at the dock wherever possible.
Every liveaboard I know - the 40 at my marina as well as dozens of others - keep their batteries on float at the dock with a marine quality charger. When 12 volt items are used the charger carries the load. It's the best way to prolong battery life.