Make sure you have the time to keep checking those batts for overcharge and be wiling to accept 90% as full. Or get something like this Schauer CM16A Charger Desulfator and get a full charge with maintenance.

I know its not multibank but I dont need multibank and not willing to pay for convenience of a waterproof marine charger when the pigtail can stay connected to the single batt. I can plug it in when home without lifting a cover. BTW if fast isnt needed, the 6 amp is only $30-40.