What charger should I use?

OK here is a question for those who are showing the math in their responses. The math that I do not understand. Since I am going from a 6345 charger powering one group 24 battery with 75AH capacity, to now having three group 24s with 225AH capacity, will I gain any advantages from using a higher amp charger? If I choose a PM-4B converter charger I have the choice of a 45, 55, 75, or 100 amp model. What do I gain with a higher model except higher out of pocket costs? Maybe reduced generator run time? Maybe longer battery life? If I was to balance the longer battery life achieved through a slow charge rate with the extra gas burned in the generator to perform this task I might come out even in the end. If that is the case then I would opt for the faster charge rate and replacing the batteries more often. More convenient for me. Can you math guys explain why a 45 is better than a 100 or vice versa?

Thanks for all your help. I am really trying to understand this stuff. My regular weekend routine with the old setup was to run the generator for an hour and a half every morning and an hour every night before retiring.Some mornings the generator would not start and I had to start the engine for a few minutes to charge the coach battery enough to get the genny to roar to life. There were other pilots at our meetups who never ran their generator all weekend. None of them had solar. Just larger battery banks.