re: " I hope that none are bad, but....." - a 12v
battery pulled down to 6v is on borrowed time if not already dead.
Put a 12v
light bulb in series with your 10 amp charger to help reduce currents as you try to bring the batteries up. Even with just 1 battery at a time, a resistance like this can be a good idea as AGM's will pull a lot of current that the charger may not be able to handle for very long.
Then invest in a converter or battery maintainer that will keep your batteries at top charge and apply a desulfation technique when you are not using your batteries for more than a couple of weeks. To get optimum life from your batteries, exercise them properly, recharge promptly and with appropriate vigor, and don't let them sit without a top charge for very long.