I should have to added to the above comments I like to make my measurements using my multimeter at the battery terminals when checking things. With the shore power OFF the battery terminals should read 12.6-7 VDC if fully charged. When I connect the shore power to the trailer then the battery terminals should read 13.6VDC (Unless in a smart-mode charging routine) telling me the converter/charger unit is working and all the connections between the Power Distribution Panel and the battery is good.
Here is a neat DC VOLTAGE verses CHARGE STATE list...
I always go to great extent to "NEVER" let my batteries get below the 50% charge state or around 12.0VDC... I also have this nice homemade panel installed inside my OFF-ROAD POPUP is a good spot to view that tells me my batteries conditions at all times.
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All of this adds to us being very successful camping off the power grids...
Just my thoughts again...
Roy Ken