Hi,
Do you know what the battery charger / converter looks like?
IF you can find a silver box, with large wires going to either the battery or more likely the fuse box, and it plugs into a 120 volt outlet, or has all the circuit breakers built in, then give us the model number from that device.
We can tell you if it is worth fixing. Probably not. Might as well just replace it with a similar amperage 120 volt in and 12 VDC (nominal) output. You should actually see 14.2 volt maximum for a few minutes to one hour, then it should reduce to about 13.5 volts for a couple of hours. After a few hours, then a charge rate of only 13.2 volts will keep the battery full while not heating it up, or boiling away to much water.
Fred.