It sounds like you already have a trickle charger, but if you're are looking for a modern multi-stage microprocessor controlled general purpose charger, I like the Schumacher SSC-1500A-CA
. It tells you percent of charge while charging, then after it reaches 100%, it will keep a top-off voltage for a while, then it will go into float mode for awhile, then it will essentially turn off, though it will continue to monitor your battery, and give "kickers" as required.
You probably already know, but I will mention it anyway: Remember that you will need to charge your batteries when connected in series as a 12V system. I don't know of any "smart" chargers for 6 volt batteries, though they probably exist.