Just another option to consider, we put 2 sealed 6 volts in place of the single 12. They fit in the same compartment (sideways) and didn't have to have a battery box as there is no out-gassing. The sealed 6 volts are pricey, but in our experience at work, the average age for replacement is somewhere between 6 and 10 years. Besides, having a battery box built would have been expensive as well. Our battery was in the same compartment as my hydraulic stuff. All we did was weld up a little more support for the floor and stuff them in. Side benefit, no maintenance.
The trailer has been sitting in the driveway for 2 weeks. Running the fridge and other parasitic draws and I've still got 3/4 battery. with the 12 volt, even at it's peak, we would have gotten maybe 3 days. With the batteries we used, we tripled our available amp hours. The good, true 6 volts are also made to be drawn down and recharged many cycles as use with golf carts would indicate. They may cost twice as much as 12 volts, but you'll probably get many more months of service from them.