Battery voltage increase after load
Some OEM GEL suppliers suggest;
After charging all batteries, if any battery voltage is still greater than 0.3V compared to any other battery in the set, you may have a failed battery.

Terms

100% State of Charge (SoC) =( equiv. to )= 0% Depth of Discharge (DoD)    

Wh Energy Capacity = Vavg * Ah = 2790 Wh  = 12.4 * 225Ah new @ 25'C

Your tests


20 watts + inverter loss?, 2 hours 12.5 volts


Bulb energy= 40 Wh = 1.43% of rated new capacity
12.5V = 60% SoC= 40% x 2790Wh=1116Wh
thus Used/rated = 40+? / 1116 * 100%
= &lt;4% of expected capacity. really dead 



There are too many contaminants that affect battery aging, but most common is lead sulphate crystals (rust) that can be ablated and settle to the bottom of the case. Some products are good and many poor that apply high-frequency (> 20kHz) high-current but narrow (&lt;1us) current spikes while drawing from power on float charge.  

Probability of success varies greatly with chemistry, pulse device, time to recovery and duration of condition. If 50% in capacity if detected early success rate can be > 50%  but after 1 yr, unlikely.

My experience is the best cure is a preventive maintenance device that operates continuously using &lt; 5W while on float charge only. ( Auto-sense)

My simple electrical equivalent circuit of a battery is below.







simulate this circuit &ndash; Schematic created using CircuitLab

Sulphation causes ESR1 to rise ( and also drained battery)
C2 has higher ESR that takes longer to charge which completes after CV is current reduces to cutoff.
C2 has the capacity to restore C1 voltage and is the "memory effect" in all batteries. ( some more than others)
But ESR2 is too high to support higher currents.



Trojan AGM REF

https://batteryuniversity.com/learn/article/sulfation_and_how_to_prevent_it

Never attempt absorption charge levels ( overvoltage) on AGM batteries.