Im not sure that im following your situation exactly, but AGM capacity anxiety is common. While AGMs are well suited to RV applications (although lithium is rapidly taking over), the trick to sanity there is a) knowing that youve fully charged them regularly, and b) employing some kind of coulomb counting based state of charge monitoring. Fixed absorb times are difficult to work with, can over charge or undercharge. A charger with true end amps will help a lot there. You dont want to under charge them because you create a spiral of reducing capacity each time you do it. You dont want to overcharge them because youll wear out the catylyst and pop the vents releasing gas that can not be replaced, causing the battery to fail early.A battery monitor like the victron/bogart's or the one built into the midnite classics will help give a reasonable estimate of SOC, but perhaps mroe usefully it also allows you to monitor in real time what is going into and out of the battery, thus allowing you to trace unusual loads. Over time you will learn how your particular batterys rest voltage maps to SOC, but for now it sounds like you need better information before worrying too much.
From what you have posted, I do not see you doing anything wrong (especially in terms of under charging).
The batteries are ending the charge with high voltage and low current. Whether automatically terminating or you stopping at low amps (down to 0.1% or below 2.5 amps), it does not explain your poor bank performance (unless you took the batteries to dead and sat for 1+ weeks uncharged, or the batteries were delivered dead to you).
One question, that you may have missed of mine (or I missed your answer), what is the voltage of each 6 volt battery during loads+ charging. Are the voltages very close or are they >>0.01 volts different.
Looking to understand if you have one failing battery or two failing batteries.
At least, one failing battery would be easier to attribute to mfg/not your problem.
is that 12.1 V been measured at rest or under load? There is a big difference...
It does take several dozen cycles +/- (deeper than 75% state of charge) to full form plates.
The wrap up to this was that I sent the batteries back to the dealer.Rolls - Surrette AGM are Full River AGM with a red case. Knowing that would have saved $50/battery....................................................................................... After the batteries where tested I sent them back to the Rolls dealer. They tested the batteries with a light then a heater and said the batteries tested great. I asked what the amperage and the finishing voltage was. The dealer said "We are not going to send you a Excel spread sheet". I got a full refund and bought a 6V open cell batteries from the dealer that tested the batteries for free.