Sounds like the converter/charger cooling fan is running full time.
The batteries have been overcharged, boiled dry.
IIWY...I'd get a new 3 or 4 stage converter, and new batteries.
The new converter will prevent the batteries from overcharging when plugged in for extended periods.
Randy @ Best Converter
is a good place to start your research, very helpful and won't steer you rong.
At the very least you'll probably need new deep cycle batteries and then test the converter for output.