You can't charge a 24V system with a single bank. Get a bank for each battery and they'll always be ready to go.

A properly functioning onboard charger (or any "smart" charger will not boil a battery during long term charging. Water loss occurs during the faster charging cycles, and it is going to happen everytime you discharge and then recharge at least a little. Chargers switch to float mode when the battery is fully charged, which is a voltage and current below the gassing level of the battery so no accelerated water loss occurs. If you have a bad battery or there is a load on the battery that keeps pulling it down, the charger may remain in the faster mode which can result in accelerate water loss.

I plug mine in 24/7 unless I'm fishing, and leave it on charge from late Oct or early Nov until March every year. Everstarts are on their 4th season and seem like they'll make it through this year fine. You need to eyeball the lights on the charger and the water level in the batteries every 30 days to make sure nothing is going wrong.