One comment I will make.. I know this sounds confusing but I'll try to clear it up.
You do not have six volt batteries in your RV.. Oh yes, there are two things there that look like six volt batteries, And in fact they were six volt batteries when they were sitting on the shelf in the store.
But in your motor home they are two halves of one 12 volt battery.
Like this
-{Bat}t{ery}+12V
Think of the two of them as a single 12 volt battery.. NOTE also this answers many questions.
For example:
Can the two six volt be different makes, models, sizes, ages, whatever?
Would you buy a 12 volt battery that was half one thing and half another?
Same answer
See how easy that was.
Mine used to be Interstates.. They are now DEKAS. Just got new ones after 9 years.