I thought I'd follow up with a bit of status.
I poked around and found no 12v coming from the converter at the main 12v distribution panel by the battery. I disconnected the charge wire and hooked up my new battery pack. I found 12v on the charge side, so the issue appears to be the converter itself.
Here's my plan:
1) eliminate the isolator pack. I'm going to use a battery combiner to allow charging from either side.
2) Replace the converter with a modern one. I want the hum to go away and since the battery charger portion is dead I have an excuse. (OK, I could repair the thing but I'm not going to.)
3) Install a luna national dual battery monitor.
4) Resize the battery compartment to hold a pair of golf cart batteries. It's too short right now. It'll be a fun welding project.
Later projects:
5) Install solar charging.
6) sine inverter - On my to do list will be a power inverter for laptops, etc.