I decided it must be the converter connection(s) at the DC fuse panel, since that is the last thing that was monkeyed with. The battery wires also go to lugs on that same panel and might even be sharing lugs with the converter's wires.
Look for a loose set-screw or a wire not all the way in the hole or with many of its strands missing. The guilty lug might be on the back of the fuse panel where it can't be seen. The fuse panel can be removed to check the back of it.
I am doubting the batteries can be that bad (to half as new) and still be working, so it is likely wiring.