Well the 5+ year old engine battery is a group 27 too. I have a hard time believing it is still viable.
I agree that the results during charging are not really making sense. Something somewhere is just not right. It is as if the batteries have a large amount of internal resistance, or some current is going somewhere bypassing the shunt.
When I had all three in parallel, and removed 33 amp hours from them overnight, the lowest voltage recorded was 12.4.
Without the engine battery in the mix the batteries hit 12.1. Note that this is not what they stabilized at, just what the ~2.5 amp cycling fridge was able to pull them down to, just before sunup.
Frankly, the underperformance of the solar is a bigger concern than the poorly performing batteries.
