One thing to remember...
It does not matter if you have 1 Marine/Deep cycle 12 volt
or half a dozen of them in parallel
Or a pair of six volt in series
Or half a dozen of them in series/parallel
Or all of the above
YOU HAVE ONE 12 VOLT BATTERY BANK
so you charge all of them the same way..With a 12 volt charger
The only caravets are:
1:Size of charger... A pair of six volt can accept 60-70 amps so with two pair (or more) you can put in a very large charger
By the same, a pair of Group 29 can suck around 30 amps each (60 per pair) so if you had half a dozen.. thats 180 amps max charge rate
And if you have a whole bunch of batteryes,, Then you run into a minimum charge rate...But with just two pair.... You are looking at around 1 amp minimum.