So the way I read your latest testing result is:
converter output connects via red wire to the left main terminal of the latching relay; black wire from the right side main terminal of the latching relay connects to the coach batteries (via a 50 amp cct breaker connected in series). So when shore AC is disconnected the red wire has no DC voltage on it because the converter has no AC input so no more DC output.....and because I suspect the latching relay has failed open.....there is no 12.6 vdc coach battery voltage connecting through the main latching relay terminals from black cable to red cable. If this latching relay is not clicking (to close and to open) when you operate the use/store switch then I think you found the problem. When the latch relay closes 12.6 vdc from the coach batteries connects to the converter output and to the coach 12 volt fuse panel for your loads. When shore power AC is connected again the converter is now putting out its own charging voltage (typically 13.1 or more depending on charge cycle needed) and “if” the latching relay is closed (use) that charging voltage is now applied to the coach batteries. Depending on voltage level of the coach batteries, the isolator relay will turn on and connect the coach and chassis batteries together to have the converter now charge both sets of batteries. I would suggest the latch relay sounds like it is open and not closing when you operate the use/store. Just my 25 cents worth......maybe worth it.
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