A couple of thoughts
Where in your little traveler will you be installing the batteries? 2 6 volt batteries need to be installed close to each other to minimize the loss to make 12 volt
The floor of the trailer wherever you decide to place the batteries will need to be reinforced for the weight. A single axle trailer spends lots of time in the air
the jarring will cause them to drop through the floor and even bend outriggers
With all that out in the open, have you considered mounting them in the bed of the truck, having 1-12 volt in the trailer with a pigtail that connects the external bank in with a relay to disconnect the 12 volt
in the trailer. This would allow you to charge the 2X6 if you do any driving around and even recharge the on board 12 Volt
from the 2X6's by connecting them all together. Just my random thoughts..............