...so it turned out that the two GC2 6v coach batteries did need to be replaced however I suspect that the converter is still the suspect in this issue.
We're just away for 3 days and the new batteries were reading about 12.45v after a 3 hour drive and after sitting plugged into 50a shore power the reading is now 12.05v. From what I've read these new batteries should be into the 13.x range.
When I last looked at the converter, behind a panel under the fridge, the OEM WFCO converter is mounted on the floor unfortunately the rear with the connections and fuses is facing the outside of the coach. When we get back I'll disconnect the coach batteries, unscrew the converter mount and turn it around and meter the output terminals. If there's no voltage coming out and the fuses are fine I guess it's new converter time.
After combing through many threads it appears a good replacement is the Progressive Dynamics 9260? Some have replaced with their 45amp version but as the wiring here should support the 60a I guess I should go with it.