Conveyor belts are designed carry packages, trucks carry pallets with 20-80 packages on them. Packages are loaded and unloaded onto transfer trucks in pallets. Conveyor belt will be useless in a truck bed. They could do one of those floors that cargo airplanes have where entire floor is tons of balls that enables you to slide anything across the area but then again special flat bottom pallets would be needed and those metal balls will make truck a lot heavier.