"I've yet to pull the 4 UB12220 bank and charge it, then test each battery
separately, but am getting symptoms that they as a unit,
are no longer holding the charge they used to, by almost 50%."

I think it would be worth checking individual batteries.  Some bad cells in there could definitely bring down the rest.  I tend to be kind of anal about my batteries and have installed a separate battery gauge that I check frequently.  Anything less than 80%, I start to get worried and investigate.  I do use a 3 stage "smart charger" frequently for battery maintenance.  My UB12220 are working great, but I've only had them install one season.  I do leave the smart charger on the batteries all winter long in my garage to keep em topped up. It has a "floating" mode it stays in.

Troy