Throttle charge current by voltage not by amperage. I have treated GC's to 90+ amperes from a vehicle alternator so many times I lost track decades ago. All the time in the world type charging, 20 amperes is plenty gentle. When time is money or someone is ready to level an aught-six at you because of generator usage, then mimic what an alternator does and regulate voltage to 14.2 and let the alternator have at the batteries. Sixty amp charge rate for GC's is ho-hum. But when connected to shore power for as long as you wish, I would throttle them back down to 20 amps charge rate.