I've typed this stuff so many times... I finally saved a copy so I don't have to re-type it all the time. This is not 100% to the question but does answer it. And the answer is:
In the RV world there are not six volt batteries
There are only 12 volt batteries
Some come in one piece, some in two pieces but they are all 12 volt batteries
How can I say this? Simple There are no 12 volt batteries either, there are only 2 volt batteries
A Six volt golf cart battery is really 3 2 volt batteries in series A 12 volt is six of them
If you take two six volt Golf cart batteries and hook the POSITIVE of just one to the NEGATIVE of the other. You have a 12 volt battery. it's that simple, Think of this pair as one battery, treat it as one battery with only two exceptions and all your questions are answered.
The two exceptions: Cleaning (Clean all 4 posts, not just the two at the ends) Lifting (When you lift them you first disconnect them, THEN you have 2 six volt batteries)
Now: This brings the quesiton "Why do folks say 2 six volt are better?" Tradition of course.
Not all that long ago you had your choice of 3 classes of batteries
Starting (Could be found in six volt, or 12 volt)
Marine/Deep cycle (I'm not sure if they came in six but they sure come in 12)
And TRUE DEEP CYCLE, which came in Golf Cart and Fork lift sizes. Golf cart are six volt and can be lifted by a healthy person, Fork LIfts are normally 12 volt by over 1,000 pounds and can be lifted by another fork lift.
Today, you can get True Deep Cycle in different sizes from small enough to fit in your pocket to large enough that you need a fairly LARGE forklift, but it will run your car 300 miles between charges.
You can also get a lot of different chemistries Flooded wet (Golf cart are usually), Maintenance free, AGM, GEL and more
But there are still the 3 main classes Here is the chart
Starting Marine/Deep Cycle True deep cycle
Lots of Cranking amps Lots of Amp Hours
Not so many Amp Hours Not a lot of Cranking Amps
You will notice that the Marine type is not a lot of anything