Update. I charged the bad house battery and got it to show borderline bad (very weak). It is definitely time to replace the bank. The inverter/charger has settings for wet, gel1,gel2, and AGM. There are instructions for checking and changing the mode in the manual, but it doesn't really work like the manual says (or at least I couldn't get it to work that way). Finally, I figured out how to check the mode and it turns out it was on Wet mode (I have a bank of AGMs). So now I have changed the mode to AGM, but I believe the damage is done.
Incidentally, the batteries were installed by a local RV shop that did all the maintenance for the PO. I have found numerous problems with their work (I have the receipts). I am using a different shop now.
So now that I can control the charging mode, I guess I am going to try KRumm's suggestion. I can get these for $900 for a bank of four. They seem to be well regarded:
They are slightly smaller than the GC2s, so they should have no problems fitting in the same space. I'll make sure they are well tied down.
I'll let you know how it goes.