We still have our original Duo Therms. They work great, but have started cutting out when it gets really hot. They'll run for 20 minutes, then cut off and the CCC thermostat goes blank like the switch is off. As mentioned earlier, this got worse on our drive home from MOT (unrelated to any repairs that MOT did).
This is typically a symptom of an issue with the CCC thermostat switch. Start there.
In our case, I replaced the thermostat, but still had the cutting off, so the next place to try was the control board in the front unit. The control board is located outside under the clamshell and a sheetmetal cover. The first photo shows the location behind the fan and compressor capacitors.
After removing the power, shorting the capacitors, removing the screws for the brackets, and lifting out the capacitors, I struggled a bit with the flimsy plastic snaps that hold the board in in four places. I carefully swapped each wire from the old board to the new board. In the second photo, you can see the wiring on the original board. Finish by placing the new board back on the snaps and press down on the board against the standoffs on the backplate until all four snaps snap into place holding the board. Restore the capacitors, sheetmetal cover, and clamshell.
I tested last night. It was 101 inside when I arrived and the ACs ran without stopping until it was cool inside.
The control boards, both old and new, have a part number of 50-981502-B stamped on them. The Dometic part number is 3311557.000 and I was able to source them on Ebay (https://www.ebay.com/itm/322632009623). They're also available at PPL Motorhomes for a bit more.
Another item knocked off my list.
We had the same issue, Just an FYI, if needed, those Boards are available on Amazon too.
Nice write up Rick! Congrats on solving the issue!