What I found when I was researching this topic is that 6 volt deep cycle batteries will come with greater ah than 12 volt deep cycle batteries. Your 6 volt deep cycles only have on average 220 ah and you don't get any benefit in increased ah when you add a second battery you just get 12 volts. In comparison a 12 volt 24 group deep cycle battery will only have on average 85 ah but you get to ad 85 ah every time you add a 12 volt battery. Two batteries will get you 170 ah.
Now if you go with four 6 volt batteries you will end up with 440 ah compared to 4 12 volt batteries you will get 340 ah.
6 volts will cost a little more than 12 volt batteries. Also when I bought my batteries at Costco 12 volts had a 2 year warranty and 6 volts only had a 1 year warranty.
6 volts handle charge and discharge cycles better than 12 volt batteries.
For me my trailer was new and had one new 12 volt battery so I elected to buy 3 more 12 volt batteries for a total of 4.
If I had to do it again and start from scratch I would buy 4 6 volt batteries from Costco... They were just under $100 each the last time I looked there is a core charge of $15 per battery...