Battery Recommendation

When batteries are wired in parallel, the amps are doubled. When wired in series the voltage is doubled.

So you can do dual 12v's OR dual 6v's..

By using them in parallel you actually get more total amps out of them, compared to using one and then the other.  With two 100 amp batteries, going to 50% SOC on each you'll get 50 + 50 = 100 amps of usable power.  Put two 100 amp 12 volt batteries in parallel and measure the voltage to get to 50% SOC for the bank, and you'll find you get about 110-120 amps total.  This is due to something called the Peukert Effect if you want to know more.

Thank you all for clarifying this.  Will go this route and try it and see how much more I can extend my dry camping time.  All I need to get are a couple of heavy cables to go from + to + and - to -.

It is my understanding that when wired in parallel, a stronger battery will drain into the weaker one (battery canibalization). And that's a bad thing.
Is this avoidable if you have exactly the same batteries purchased at the same time?

Originally posted by howmanyds It is my understanding that when wired in parallel, a stronger battery will drain into the weaker one (battery canibalization). And that's a bad thing. Is this avoidable if you have exactly the same batteries purchased at the same time?

If, as you say, both are exactly the same and purchased at the same time, both SHOULD be in equal condition already. Charge each one fully and check the voltage or take them and have them tested to let you know the condition. If both check out, then you should be okay.