Nice job of troubleshooting. You've already eliminated most of the things I was thinking could be wrong.
Let me be sure I understand. In the past, with this TM and this tow vehicle, you have been able to travel with the TM refrig running, and at the same time a separate thermo-electric cooler running? That is an impressive electrical load - most rigs won't handle it.
So the TM battery is connected to all the TM loads, and these loads are sucking the TM battery down. And power from the tow vehicle reaches the converter, but doesn't pass through, to power the loads and keep the TM battery charged? This should be a direct metallic connection - to my knowledge, there is nothing inside the converter that could affect or disconnect the pass-through of the truck power. So that is where you should start looking.
First step. With everything hooked up as it should be, tow vehicle running, use you meter to measure the voltage between the converter's 12-volt in (from the tow vehicle) terminal, and the converter's 12-volt out (to the TM battery) terminal. There should be no voltage there, but it sounds like you will find almost 12 volts there, indicating an open or almost open circuit within the converter. Assuming you find a big voltage there, next step would be to shut everything down (disconnect TM battery, disconnect Bargman connector, no shore power), disconnect from the converter terminals the two 12-volt wires mentioned above, and connect them together to bypass the converter (be sure they don't short to anything else, including ground). Start the tow vehicle, plug in the Bargman, and see if the situation is now normal, or still has an error. If you can do it, the best check is to measure the charge current into the TM battery with the tow vehicle running. It should be a couple amps with no loads turned on and the TM battery charged.
Bill
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