You first need to charge them to true full. Then do a "recondition" charge.
To get them to true full, you need a couple days on shore power so your rig's 12v will be from the converter and nothing from the batteries.
Now you disconnect the pair of 6s and charge them with something that brings them to 14.4-14.6 ish volts and holds that voltage for an indefinite time.
Use your monitor or another ammeter to observe the charging amps taper, once they start doing that, until they get down to 0.5a/100AH, or 1.5 amps in your case--if amps go down more than that then let them keep going until they bottom out.
Now they are full. So the overcharge will be to crank the voltage up from 14.4-6ish to 15.x and leave them doing that for a while, observing casing temperature with your IR gun. Temp will rise slowly. If it starts going up faster stop. Otherwise give it a few hours, then stop.
So you are asking, howinheck can I keep the voltage constant to do that when my converter or charger drops to 13.6 all by itself before the battery is fully charged?
A. if you have a PD converter with charge Wizard, just keep hitting the boost button as required to keep it at 14.4.
B. own a charger with manually set adjustable voltage that you control. Example:
https://www.boatandrvaccessories.com/products/powermax-pm3-60lk-12-volts-60-amp-power-converter-battery-charger-w-led-lightAssuming your AGMs are recovered ok, now you have to keep them that way, by getting them to true full as described above, as often as possible. Follow the AGM guide in here even if you have some other brand.
https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf
