I used this same switch in my previous SOB. In that case I had two 12 volt
banks, each made of 2 6 volt golf cart batteries. If I was connected to Shore power or to the TV, I put the switch in the BOTH position to keep both batteries charged. When not on shore power or TV, I would run off one bank at a time, so I would have a good idea how much useful time was left after the first one discharged. I also had solar panels on the roof and another smaller switch to independently switch them from one bank or the other or OFF.
It was an interesting and expensive experiment, that I would not do again. It is a lot simpler to stick with as many 12 volts batteries as practical in parallel and keep track of their state of charge by monitoring their voltage. If you suspect one is going bad, just disconnect it until you can replace it.
I got lost a bit in the calculations in the preceding posts. Just to clarify: two 100AH 6 volt batteries in series = one 12 volt
Also, contrary to stern warnings previously posted, connecting different capacity but same type
and voltage batteries in parallel is not the most efficient way to operate, but it will work fine and will not ruin either of the batteries.
P.S. Keep in mind this is hopefully just camping, where you can pick up and leave if something stops working and not life and death survival. In that case all the rules are much more strict.