Ah, another dreaded WFCO 55. These are notorious for not going to boost mode when expected such as batteries down 50%.
Give it a test before you spend money but I think you will want a new converter if you expect a decent charge from the generator. Turn a light on until battery gets down to 12.2v to 12.0v zone. Then run the generator while watching voltage. Battery should climb to 14.0+ volts within 30 to 60 minutes and you are good to go. If battery is 13.6 max then you have no boost charge and it will take 8+ hours for a 90% charge.
IOTA, PowerMax, Progressive Dynamics are the best replacements. These charge at 14.4+ volts every time to get you to 90%+ in 2 to 3 hours.