Thank you Libero. I had to leave this for a few and got back to it this weekend. I turned on all the interior lights with the batteries reconnected to draw them down from full charge to see it the converter/charger was giving me 13+V indicating battery maintenance, like we discussed. I was tidying up the inside and heard the fan in the converter kick in - I was quite surprised. I isolated the batteries again and low-and-behold, the 12V stayed on from the converter - it's back!
Looks like your theory about the batteries being too depleted was right and once charged again the converter reset itself and came back online.
The lead-in to all of this was our coach had been in the repair shop for three months after a disagreement with a gas station this summer that almost tore the rear wall off our RV. The original estimate was a three-week repair but they kept it for three months. It would seem that whatever they were doing with my RV all that time (supposedly waiting for a replacement bedside cabinet from Winnebago) drained the coach batteries completely at which point nothing would latch on, not even the isolation relay.
Thanks again folks, especially Libero, and hopefully this post helps someone else in the future.