But don't overfill batteries either. Escaping electrolyte causes corrosion. Keep batteries free from corrosion. And make sure connections are both clean and tight. And wear safety glasses.
Alternator charges both house and starting batteries.
Water replenishment requirements may be anywhere from a few weeks to a few months. Every time you discharge and recharge, there will be some gassing, which diminishes the electrolyte level. Dry camping will cause water consumption to be much greater than if the batteries are plugged in and getting a float charge for maintenance. They are expensive, but offer a promising new development in battery technology. Typically if you need to replace on battery, it's time to replace all of them.