In California, if you have a bed on your truck that is not the pick-up box, you are required to stop at all scales. This includes flatbeds and utility beds. It makes no difference what the GVWR is. You are required to register your truck, flatbed or pickup box, with commercial license. If you wish, you can get non-commercial plates, but you will be cited if you put anything in the bed of the truck. You can register your truck as a motor home, but only a TC can be in the bed, and it is supposed to be fixed (bolted). If you have never heard this one, you may want to check with the California Highway Patrol, or the California Vehicle Code Book.
Wayne