AGM batteries in parallel is a tough cookie. Their internal resistance is so low, that the cable interconnects are not going to help share the load. So the best battery is going to contribute most of the work till it's no longer the best, and another battery takes over.
Overall. massive parallel banks tend to not last as long as series banks for the same batteries.
Adding new batteries to an old bank, usually just rapidly ages the new ones til they are a bad as the old ones.
If you must parallel batteries, this website
http://www.smartgauge.co.uk/batt_con.htmlhas a good tutorial on how and why (with all the math) so you can get the most life out of your expensive batteries
