The 4x4 isn't a dumb idea because of its strength. It's more than strong enough. It's a dumb idea because there's no way to rigidly secure it to the truck frame. Mount it with wire or galvanized strapping and that beam will move around every time the suspension flexes, loosening the straps all the while. The mounts need to be BOLTED to the frame and you need metal tie downs. Fabric and rope straps degrade in the sunlight and weaken with dust and grit in the fibers. They also may stretch when wet from rain, loosening as they stretch.
Get the right stuff. Every else on the road deserves for your 3000#+ camper to be secured safely.