I'm using two 6v batteries, new last year. I maintain them as I do the golf carts I maintain (several). Thus far, they are testing ok, but to make sure I'll put another set in to verify. It is grounded to the frame with about two feet of wire, and I did check that to make sure it was a good connection, etc.
I unplugged the converter and disconnected the positive on the first battery. There are three red wires terminating into the 12V power panel. I disconnected all three of those and toned them out. One goes to the converter. The other two seem to go to a solenoid by the chassis battery. Not sure if that is the battery disconnect or the emergency start solenoid. There are two connections to the positive on the battery. One of those also tones out to the same place under the hood, but I have not yet located the other. I get a very quiet tone near the battery disconnect switch. I'll go crawl around underneath it and see if I can see where it goes.
It seems odd that two wires would terminate in the same place, no?