Battery advise needed

If you put two batteries, 2 6 volt in series, or two 12 volt in parallel make sure both are close as possible in build dates, (ideal would be to have two batteries with sequential serial numbers i.e. ...1 & ...2. If they are not perfectly matched the weak one will bring the other down to its level. Also, do you do a lot of dry camping? If you only go from plugged in camp site to another plugged in camp site IMHO you do not need to hassle with two batteries. One gel cell will be long lasting hassle free alternative. A little pricey, but one gel would be cheaper than two lead acid!

They are taller, about 1.5 inches. They are about the same rectangular shape. Slightly shorter in length and same width as group 24.

I plan on many unplugged days, so looking at 6v seems logical.
I am going to go over the whole setup today with a meter and determine if the 12v are even in right.
Are 6v just taller than 12's or bigger all over? NP if they are just taller.

What kind of camping do you do? If you always camp where you can plug in then big, multiple, or fancy batteries are a complete waste of money. Just get the cheapest group 24 you can find.
If you do need some battery capacity then yes, 2 6v in series will be better than 1 or 2 12v batteries. If that's not enough capacity, use 2 sets of 6v for a total of 4.

6-V golf cart are great - but you need to find a vented battery box to put them in, because they are too tall for the standard type boxes. I ended up going to a blue sea marine box and modified the top and bottom to accept the vents.

I was too lazy on this rig, and just went to the 2 27 series 12V, as it came with two battery boxes already.

Even old batteries should have charged while plugged in
IF
They aren't shorted...bad cells etc
Converter is functioning....good AC power in (AC circuit breaker ON or outlet it is plugged into has AC power). The reverse polarity fuses (2 large fuses on converter) aren't blown for hooking cables up wrong when swapping batteries.
And battery disconnect switch is in proper position.

Take it you have dual batteries?
In separate battery boxes?
May not have room for 6V batteries. They are larger than 12V

Check that batteries are even getting a charge......converter output should be 13.6V DC with batteries disconnected
Then check the reverse polarity fuses
Then check battery ground...clean tight connection to frame

And you can get batteries load tested at an auto parts store.

6V batteries give more amp hours IF you have room to install
Great for boondocking
BUT if you don't do much boondocking then the dual 12V are just fine

Read The 12 Volt Side of Life. It gave me a better understanding of batteries.

Two six volt batteries wired in series. Gives more power for longer dry camping.
I have seen where previous owners have swapped good batteries out and have put in their old batteries. Many dealers do it as well.