How Do I Charge, and Maintain 2 New 6 Volt Batteries???

You can discuss 6-12 all day long, and at the end of the day, amp hrs really are amp hrs, an there ain't a dimes worth of difference tween 6-12 w/ all other things being equal.
I ran 12v for many years, then changed RVs, and ran 6v for 9 yrs, and now changed RVs again, and now back to 12v batts. I'm tellin ya, there still ain't a dime worth of difference in amp hrs. I never noticed any difference.
But, I never had 12v batts last 9 yrs like the 6'ers did.