Simply put I will draw an ASCII picture
-{6v}+~-{6V}+12 volts
so your black lead to the - on the left in this drawing
the Red to the +12
(The "~" sign in the drawing is a jumper wire)
That easy
And though they are indeed a pair of six volt batteries when you are lugging them outside the RV. Think of them as a SINGLE 12 volt when installed. for that is what they are..
Many questions like "Can I replace just one?" are answered if you think of them as a single battery (Can you replace just half a 12 volt battery?
See how that works
Can they be "Different" (Makes sizes ages) Woudl you want that a half and half 12 volt battery?