How do I charge them? Well, you dont', you charge IT.. I will explain.
I have occasionally been flamed for saying there are not six volt batteries in RV's. Yes, I know that is what you have, it's what I have but they are only six volt batteries when they are OUT of the RV lying on the bench or on the shelf. in the RV those two batteries are each 1/2 of a 12 volt battery. And that is what you charge, the 12 volt battery they make when properly connected.
Think of them that way and many questions go POOF.
This is a 12 volt battery: -{Battery}+
This is what you have -{Bat}t{ery}+
in both cases - to + is 12 volt. (Each set of brackets - one "Piece".
