I have two 6 volt batteries wired in series to create 12 volts. Batteries are not charging on shore power. When I disconnect the wires coming from the converter, I get 13.6 volts. This suggests the converter is doing its job. But when I connect those wires to their posts, I only get 12.3 volts at the battery bank (same as when the batteries are isolated from the camper). These batteries are less than a year old, and nothing about the wiring has been changed since our successful summer of camping, when everything worked fine. I have cleaned and tightened all connections at and between the batteries. What else could be preventing the batteries from charging?