Hi
If the batteries are wired correctly, the voltage on the two of them should be identical. That's not to say that either one should be at 8.x volts. It's just that there should be no difference between them. The + on one should have a cable to the + on the other one. The same thing goes for the - side of each of them. Yes, this assumes you have
12V batteries. If they are 6V units, you have a very different problem.
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Charging wise, first thing to check is the breaker into the converter / charger. If it's tripped, reset it. If it does not stay reset, that's not a good sign ( = either breaker is shot or the converter is shorted).
Next thing to check is the output voltage on the converter. When the shore power is "on" into the converter, you should see about 13.6V at the output. If all you ever see is 8.x volts at the converter output, it's broke and likely needs replacement. Stuff like that happens.
Assuming the converter does come up to 13.6V, check the battery voltage. If the converter is at 13.6 and the batteries are at 8.x V, then they aren't connected to each other. One possibility is that one of the wires to the battery didn't get hooked up when the batteries were replaced. Another is the setting of the use / store switch. Change it's setting and re-check things. If you now see same voltage on converter and batteries ... that's the magic setting you need to use.
Yes, that sounds like a lot. It takes longer to read it all than to do the checks ....
Biob