The thing that got me exactly the same as you... I had four days of plug in camping, and then boondocked and had no battery power.
I ended up wiggling the wires from the converter output, and then all of a sudden the DW says, "Wow, the lights just got bright!" I had my culprit and just had to tighten up the screw holding the negative return wire.
The thing that helped me figure it out was when I was driving, the batteries charged, but when I was plugged in they didn't. I checked the converter output at it's output and it looked good, but at the battery there was almost nothing.
