As said above, basic sheet metal won't be particularly structural, although if you had drive over fenders of one type or another the could be structural.
What does the frame look like around the axles?
My utility trailer is one of those that has double thickness from the tongue back to the front axle, then drops down to a single (heavy duty) angle iron from in front of the front spring perch to the rear of the trailer. When I get around to cleaning up and updating the trailer, I'm going to add something like 4x6 x 5/16 angle iron or so the length of the fenders. with the 6" pointing up. It will be able to be largely hidden, and toughen up the rear of the trailer considerably. Then I will extend more frame to the back.
Nonetheless, that doesn't appear to be common with car haulers.