So if it is strong enough for a car to slam into the back of it and keep your rig from blowing up like a ford pinto it's not Strong enough to pull a hundred pound trailer I guess!
You're talking about 2 totally different functions, and therefore two totally different designs. The bumper is designed to survive a 5 MPH
impact (yes, that's it - that's all they're required to do per DOT standards), it's not designed to save the fuel tank. The boxed frame is designed for that. There's also a difference between
impact, and
towing (pulling, twisting, and lateral movements).
There's bolt holes on the bottom of the bumper for bolt-on tow packages. Use them! They're specifically designed for what you want to do. Why all this effort on designing a work-around?