OMG - 19 years rv'ing and been working on vehicles since a kid. But all things these days on trucks and cars is 12v. These are, indeed, 6v in series. Ask me if I am kinda embarrassed! OK, I admit was wrong and promptly as possible admitted it.
Moving on. Checked batteries individually just now. Both at 6.3 volts. When I brought them in yesterday they (now apparent) they really did not know squat or they were trying to sell new batteries. They put their tester on them and said both had no volts and no amps.
So, if the batteries are good (I am assuming, they are Deka) then I presume the converter is good.
So gotta be something else in the system - water heater or something there. But not sure why the panel shows 2 lights out of 4 for the batteries.
Anyhow, that is it. Sorry, guys. My head is hanging in shame - kinda.