An RV battery is a collection of lead-acid cells... which is why they are called batteries in the first place. Each cell, when fully charged, will produce close to 2.5 volts. It takes 6 cells connected in series... negative to positive... to make a 12 volt battery which when fully charged will produce approximately 12.6 volts. Of course it only takes three cells to make a 6 volt battery that, when full charged, will produce about 6.3 volts.
It doesn't matter how big the lead-acid cells are they will still only produce 2.5 volts. You can see this for yourself if you compare a standard RV/marine 12 volt battery to a standard 6 volt golf car battery. The two batteries are roughly the same size and weight but the 12 volt battery has 6 cells while the 6 volt battery three. We can assume the cells in the 6 volt batter are larger and contain more lead than the cells in the 12 volt battery.
The larger cells in the 6 volt battery allow it to provide electricity longer while maintaining a usable voltage and that is partly why many RVers choose to use them. Since RV circuits operate on 12 volts, an RVer who chooses to use 6 volt golf car batteries will need two of them connected in series... 6 cells x 2.5 volts = 12.6 volts... just as the individual cells are connected in series.
It's important to note that while connecting two 6 volt golf car batteries in series will double the voltage it does not increase the amount of time the batteries will provide usable power. That is determined by the size (amount of lead) of each cell.
A parallel arrangement connects all the positive sides of each battery together and all the negative sides of each battery together. When connecting two 12 volt batteries in parallel, the voltage stays the same but the battery bank will provide electricity longer while maintaining a usable voltage.
The critical difference is that series batteries add voltages together at a common current while parallel batteries add currents together at a common voltage. This is important because lead acid battery energy capacity is significantly influenced by the current drawn from them and not by their voltage.
I discovered this information the hard way when I was heading up to Edmonton last March. The weather was freezing (-12 C) and my furnace drained the battery. The end result was that Jan and I spent a freezing night in a turn out in Red Deer..... not an experience we would like to repeat.
On our return to Raymond, I bought 2 x 6 volt batteries and wired them up in series. I had to make a few more mods like replacing the battery tray and replacing the gas pipe with a longer piece. It was completely worth it because now I can dry camp for 4 to 5 days with out worrying about loss of power. I also carry a spare power supply which gives complete peace of mind as it also has a set of attached jumper cables just in case I happen to drain the Tow Vehicle battery too.