Follow-up for my issues in case someone later finds this thread...
- Replaced power converter with a new 45 amp unit (PD9145A). It was a direct swap.
- Replace both batteries with new
A/C has been working fine since yesterday evening, along with the fridge and all other 120V appliances. All 12V circuits are good and the lights are nice and bright.
It appears that this was a 3-part problem. The (2) 6V house-batteries going out was one problem. Me trusting that my friend had a 120V circuit to plug the RV into and not a 220V circuit like I asked of him (I should have checked) was the 2nd problem. The 3rd one being that, because of #2, I probably hurt the converter, which further caused the demise of the batteries.
I consider myself lucky in this endeavor. All told, this has only cost me about $500 in parts. Of course my labor is free!