I'm not sure what battery box you are installing, but Torklift has the PowerArmor that also has a trickle charging solar panel on top to keep your batteries up when the trailer is not in use.
Although I like the capacity of dual 6 volt batteries, I don't like the fact that if one fails your entire system is down. If you were setting up a bank of four, then you still have redundancy. Since the batteries are easily accessed on the tongue, I don't see a need to buy sealed batteries - You can buy larger flooded cell batteries and just make sure you check the water levels.
If you leave your trailer plugged in all the time, replace the converter with a better one that has intelligent phased charging modes. This is isn't a requirement if you don't have constant power, but it will help restore a charge on your batteries better than the OE model that came with your trailer when you boon dock and need to run the generator.