Hook them up exactly as they are hooked up in the RV
And then you charge the resulting 12 volt battery
Just like you do when they are in the RV
I'm fond of saying there are no six volt batteries in RV's.. The reason for this is when you put two six volt batteries in series, as I did above.. you wind up with, electrically and electronically, one 12 volt battery.
Think of them, when so wired, as a single 12 volt and all your "six/twelve" questions are answered..
Examples of such questions:
Can I replace just one of the six volt batteries?
(Can you replace just 1/2 of a single 12 volt battery?)
How do I charge them (you charge IT)
Do they need to be the same age/make/size/model/ and so on?
(Would you buy a 12 volt that was half Interstate half Excide?)
(Could you find such a battery?)
As you see. when you think of the pair, in series as one 12 volt battery many questions answer themselves.
Home is where I park it!