We installed a commercial marine temperature compensated charger on our last boat in 1998. The present owner just wrote me (among other topics) to say the batteries continue to deliver full performance.
What ever you do in this situation it is a good idea to back up and ask why you are having to do it. AGM batteries are very sensitive to charging regimen. Done properly they will deliver excellent service for 10-15 years. Charged improperly they will fail to deliver even half the expected amp hours.
First, these batteries were designed around three stage charging.
Second, the voltages (and temp comp) for charge, absorb, and float phases are critical.
Third, they must be recharged 100% on every cycle.
That last item is critical. No deep cycle battery should ever be discharged more than 50%. Many people with a wet cell background have become accustomed to considering a battery charged enough at 85% -- about the point reached when the long absorb/float process begins. Wet cells tolerate this reasonably well. AGMs do not.
AGMs treated this way will recover less and less of their performance on each charge cycle. It starts slowly, but within a season or two, a 600amp hour bank can be delivering less than 100 amp hours before reaching a discharged voltage state.
For this reason if the engine hasn't brought the batts back for us when we shut down, the generator is fired up until all the amphours are returned (Via Xantrex).
I recommend Steve D'Antonio's AGM article published in Ocean Navigator for a fuller description.