AGM's may not go dead very easilly, but if they do, you have to have the capability of "equalizing" them to get them back. When AGM's go dead, they will charge back up under normal charge parameters just like they normally do, slightly faster maybe, but they will not hold that charge. I.E., their true capacity will be greatly diminished. If you find that the batteries reach 13.1 volts and yet as soon as a load is put on them they start dropping down much faster than normal you need equalization. Oh, yes, equalizing a sealed battery is not a good thing to do, but Lifeline will heartilly recommend it as soon as they hear what you have to say the problem is.
15.5V for 3 hours is an AGM's equalization parameter. If you have a newer Link 2000 with an AGM battery selection (#3), then it "may" have this parameter in it. Some do, some don't. If not, set it for Lead Acid (#1) and raise the ambient temp to max... 120? and let it run for 3 hours. That will do 15.75V which is ok. This is of course, AFTER you have charged them normally and they "show" full charge. Now, with the Link 2000 and Heart
Inverter/Charger, and access to shore power, this is easy. Since cruisers "often" run batteries dead, accidentally of course, and do not always have access to shore power, you should consider having the capability of putting out that 15.5V off your alternator so you could accomplish this if you needed to away from the dock.