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.
All of this adds to us being very successful camping off the power grids...
Just my thoughts again...
Roy Ken