Let me make the charging issue very very very very easy for you.
You used to have a 12 volt battery bank
You now have a 12 volt battery bank
NO DIFFERENCE (well one slight difference, it's much larger)
The only difference is that it will take your converter (Same converter) longer to charge them than it did the older, smaller battery bank,
In fact.. THERE ARE NO SIX VOLT BATTERIES IN RV's. only 12 volt
In your case (Two six volt batteries in series to make one 12) each battery once installed is 1/2 of a 12 volt battery,, it magically becomes a six volt when you remove it, but again is half a 12 when installed.
What you have there is a big (4d or 8D) 12 volt battery. Electrically that is.