Did similar to my previous travel trailer. Had four 6-volt golf cart batteries centered for weight distribution, then used the space on either side for storage of Linx Levelers, wood blocks, and other leveling materials. Nothing that might be corroded by the battery acid fumes. I also drilled a series of ½" holes on the backside, and siliconed a small piece of screen over it (to keep bugs out), so that the compartment was vented.
Lowe's has a toolbox designed to go across the front of a truck bed rather than a side box, that seemed to fit better, so that's the box that I used...
Be Safe, and Everyone Goes a Home,
2014 RAM Laramie 2500 w/6.7L Cummins Diesel and Automated Safety Hitch
2016 377RLBH Jayco North Point
300W of Suntech Solar w/Rogue MPPT Controller & Xantrex 2000W PSW Inverter