I can make it much more simple yet.
Forget all that 50-80, or 50-90 mumbo jumbo etc.
Just install a combo amp/volt meter inline w/ the output of the converter.
Fire up the generator,.......the discharged system (voltmeter on 11-12v) will start out charging at near the converters maximum amperage, (ammeter) and start to taper from there.
As soon as the ammeter gets down to 20 amps, shut it down.
If the genset is small, and you don't mind listening to it, you could run it till the ammeter get down to 10 or 15, but like said, that will take a lot more time.
While doing this short cycle living on generator, batteries will loose some capacity over a week or 2, (doesn't harm the batts) so whenever possible,... probably when you get back home, get connected to shore power for 100% charge, and the batts will be right back to full capacity.