What is the point.

If you're checking and replenishing water 2 12v will have 12 caps. 2 6v will only have 6 caps.

So insignificant, that would not even be a minor issue for me when choosing between 6v verses 12v batteries.

6v charge faster, last longer. But, in the unlikely event that one fails, the other is pretty useless by itself. For me and my style of camping I went with 12v instead.

And that is exactly why I stick with duel 12v batteries.

6v GC batteries are made for deep discharge time and again. They are built heavy to deal with it. They will last longer.
From an electrical standpoint 1 series string of batteries is better than multiple parallel strings. Parallel strings rarely charge and discharge evenly due to differences in the wire length, terminal resistance, battery internal resistance, etc. 2 6v in series will always charge and discharge evenly.
If you're checking and replenishing water 2 12v will have 12 caps. 2 6v will only have 6 caps.

6v charge faster, last longer. But, in the unlikely event that one fails, the other is pretty useless by itself. For me and my style of camping I went with 12v instead.

I've had to retrieve a battery from my TC into the house to run my CPAP during a power outage. Hard enough to drag a single battery in, much less two. Also I can use a 12V to sub for my trolling motor battery in a pinch....fishing IS my priority you know. ;)

Costco here have 6 volt golf cart batteries for about $80.
Advantage is thicker plates that are farther apart giving better cycling.