6V Battery Up-grade

follow the diagrams posted here
TIP
connect jumper wire between (1) pair in series (aka make one 12v)
then do the same to the other pair of 6v (make 12v #2)
then wire the (2) new 12v packs in parallel (ignoring the middle jumper connections)

a 12v battery has (6) cells, a 6v has (3) cells
your new 12v battery has (6) cells house in different boxes, (3) in one box (3) in the second box , each pair of boxes is (1) 12v battery and is treated just like it only had (2) ends/posts

Hook them up carefully. Double check diagrams above. You'll be fine.

My 2000 model year FW had room for 4 group 24 batteries originally. I rebuilt the battery tray so that the taller 6 volt batteries would fit. I now use a floor jack to raise the whole pack of four 6 volts into place because they couldn't go in through the small door. I installed a residential frig last fall so I added two more six volts for a total of six and added more solar for a total of 1000 watts. The last 2 batteries that I added now ride in a carrier that is bolted to the frame just behind the rear axle. I'm going to make new cables for everything this summer so all the lengths will be equal but so far it's been working well.

Your batteries will do better while under your care instead of letting them sit. Next winter while in Florida, if you need a battery in your rv just add a group 24 if you are plugged in all time. It will suffice if you need to move the rig. If you place four of your 6 volts in the original holder and add two more elsewhere you will have to consider the distance between batteries and use large enough cable. I have seen people on here mention that they have had issues when running six 6 volts so they may be able to tell you what they have experienced. They are probably not wired in a balanced manner and that's the issue I need to address with mine. My specific gravity checks ok on all batteries but I will be rewiring them.

The below is taken from this website.

Wiring Batteries in Series/Parallel Combination

In a Series/Parallel Combo Configuration the batteries are wired per the diagram below and the result would be a doubling of the voltage and doubling of the capacity. In our illustration we show four (4) 6V batteries with 225AH wired together. Each set is wired in series creating 2 banks, then the 2 banks are wired together in a parallel configuration. The result would be a battery bank that produces 12V and 450AH.

Barney