I believe your initial thought of needing a BIG charger is correct as you want to charge in a short time of running the generator
, smaller chargers are fine with shore power
You can buy a couple of smaller chargers though and that is the same as one big charger, yet it gives you redundancy in the event one fails.
I do not believe using your main engine
alternators is a good charge source, first it's inefficient and not the best way to put hours on your engines, secondly its my experience that alternators do not make rated power for long, seems they heat up and cut back on power, and most likely yours don't have good three stage controllers, they are most likely "dumb" regulated meaning they make 14V regardless of battery state of charge.
What I have done is I have this battery charger that is a 60 amp charger
And I have an inverter
that will power my AC loads when the generator
is off, but it is a 125 amp charger when the generator is on, this give me 185 amps of charging.
there are other ways of course, but I chose this way as I have redundancy and I have a source of AC power from my battery bank