You might want to figure out WHY the batteries ran dry in one yrs time before switching to AGM or any other battery.
Sounds like your converter is way overcharging, water usage with a properly working multistage converter/charger should be minimal.
If converter is not switching to storage voltage of 13.2v it is going to be hard on any battery.. Would hate to see you dry out AGMs which is a very real possibility.
In a yr and a half, my pair of 6v GC2s only used 16 oz of water and that is with the converter plugged in and hot 24/7 while not camping (and we didn't camp this yr)..