On my trucks, the frame mounted boxes held tools, as well as spare parts and my extra fluid bottles. I had a box that sat between the frame rails just above the driveshaft. It was just 10 inches deep. This replaced the deck plate behind the sleeper I kept things like bungee straps, OVERSIZE LOAD signs, flags, and other miscellaneous items In that box. When I had my flatbeds, there was a 5 foot long box between the axles on each side. Those stored my tarps, straps, my winch bar, and all my flat bed equipment except chains and binders. Chains and binders were stored in my headache rack. On my 379s, the storage box below the passenger door held my snow chains.