[JRTJH]

Who is to say that the rear bumper was welded to the frame level? Who is to say that there wasn't a loose screw or a wire laid under the floor that "jacks up" one side of the floor in one location. Who is to say that the tongue is "perfectly level with the frame 20' back from the point of measure or that the refrigerator was set in the cabinet "perfectly level with the rear bumper or tongue....

You're "assuming" (look that up if you need to) that the trailer was constructed on a jig where every component is aligned and level with every other component... Do you really believe that could happen, even if it was intentional???? Heck, most subsonic airplanes aren't built to that kind of "exacting standard"....