Assuming you still have the stock converter (the Xantrax) you can:
- With shore power connected, check the output voltage on the converter where the 12V circuits connect. It should be 13.4V or more.
- With shore power still connected, check voltage at both sides of the two red plastic covered self-resetting circuit breakers behind the battery *AND* the voltage at the battery. All readings should be 13.4V or more.
- Remove the shore power, wait 15 minutes for the surface voltage to equalize, then check the two self-resetting circuit breakers and the battery again. They should now be around 12.4V.
If all checks out, take the batteries to someone who can load test them.