8D House Battery Dropping Load Quickly

[Passages]

Experiencing a bit of weirdness regarding my house battery's.

I have (4) NAPA Commercial lead/acid 8d battery's to service the house systems. They are about 2.5 years old.

Batt 1&2, which share a common "Off,1,All,2" switch seem to drop the load very quickly, especially when handling high demand such as windless and davit operation.
Batt 3&4, which share a separate switch perform well.

I ran some tests after bringing them to a full charge and allowing the surface charge dissipate. (With batt switch off) Load tester indicated the batterys are within acceptable range and Multi meter indicated about 12.6 volts before and after load test.

Puzzled, I switched on Battery 1, and turned on some DC lights, the water pump and head vacuum pump. Within less than a minute the DC Engine room lights began to dim with each stroke of the vacuum pump.

Thinking the problem may be a bad ground or parasitic load somewhere upstream of the battery, I used a jumper cable from batt 4 to batt 1. Lights and vac pump immediately perked up. (So it's not due to an upstream parasitic load)

So, Battery's 1 & 2 have little grunt despite testing OK and only being 2.5 years old.

What am I missing?

ps: I have no problem replacing the battery's if need be, just trying to figure out what's going on.

Edit: I know someone will ask this. Yes, the electrolite is at proper level, no there is no visible damage to battery case and yes, I shined up the contacts and tightened them good before the tests.