12v vs 6v batteries

Having just done this, the biggest problem I ran into was coming up with suitable hold downs for the 2 6 volt golf cart batteries (you will NEVER regret doing that upgrade!!). My previous 12 volt "deep cycle" (ha!) was a maintenance free with a flat top that was held down with a steel plate across the top. The 6 volts don't have a clear shot across with the caps and the terminals in the way. My battery tray had a lip at the front and the piece below runs across the rear edge of the batteries and you need to run threaded rod on each side at a 45 angle down to the tray. Don't buy the "j-rods' they sell, they are too short (they expect a bracket the golf carts have). In my case the lip keeps the batteries from moving forward, the hold down prevents rear motion, and the threaded rods prevent sideways motion. Actually a much better solution than the original 12 volt hold down. If the batteries are in a difficult place to see the tops, get a battery watering kit. So much easier!
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What we did to help balance out having a good time camping not worrying about would we have enough power to run the lights or frig the next day or bring the slides in was..I installed 2-6v costco batteries as our primary batteries and kept the 12v that came with our camper as a back up battery..so in our battery compartment I use a battery selector switch so I can choose which battery bank to use 1 (6v-2)- 2 (12v-1) ...works great plus added a solar panel on the roof to keep my batteries always charged and ready for our trips....enjoy

6 volt batteries have plates that are much thicker than 12 volt batteries. The thicker plates are more rugged and hold up better to all sorts of abuse.

The cost benefit balance is a fine line. Usually a single battery will last two days in the summer, one in the winter. If you don't camp without power, you do not need a second battery at all. Most of us like to have two batteries, that will usually give you three to five days if you are not feeding power hungry children or other needs. It's really worth a lot to have a simple voltmeter on your batteries, and learn about how they discharge over the time you are using them. Helps a lot to know when to recharge, and when you don't need it at all.

Cost effective is to just add a 12v in parallel with what you have.
When both are shot in 3 to 9 years then decide what to replace with.

No, I don't dry camp that much. I'm trying to find the cost/benefit balance. I can fit 2 batteries with only the mods needed to add the second. I'm looking for the most cost effective addition

Golf cart batteries at the end of the day will give you more useable amps.
Unless your willing to spend big dollars you will not get a true 12 volt deep cycle battery. What you get for a hundred dollars at WalMart etc is a dual use battery. With golf cart batteries you get heavier plates for more deep discharges with less damage, which equals longer life. But to get the use your thinking about you will need 4 or maybe 6 golf cart batteries. Do you have that much room to spare?
Also, unless you do a LOT of dry camping, and have a means to recharge them do you really need that much power?
It might be more economical to to change to LEDs, and pracrice better power managment. Just saying.

More plates does not equal more amps. The Trojan T1275 is a 12 volt battery with 150 amps. The Trojan T125 is a 6 volt battery with 240 amps and smaller. But two T1275s would store 300 amps in parallel while two T125s in series would have...240.