I just noticed something. If the distance between the main beams or support rails of the vehicle are only about 36 inches, then the real issue is the cantilever of the overhung frame members outside those rails. That is where most of the load is and that is where the real support strength needs to be measured.
It seems simple to reinforce the fifth-wheel hitch area because it was always designed to attach directly to the main vehicle frame rails anyway with simple angle iron pieces. That stands alone and does not have anything to do with the bed. Even the fuel tank spans the main frame rails on its own.
Therefore, doesn't the weight of the toolboxes and other attached items to the overhung area become the real issue here?