fifth wheel hitch installed 4 inches back

It should be put in properly to begin with. Over the axle. That is the way they are designed. Mechanic either does not know how to do it, or the the kit was not the correct design. If it was a custom rail kit that is a no drill kit, they typically can only fit one way. Without seeing the work, I would think the mechanic did not have a clue or the directions were too hard for him to understand. Me and my father in law installed a rail kit in his 2010 Ram 3500 and the directions show you exactly where to put the rails. Gives schematic so you can measure exactly. Even if he does fix it, you will now have more holes in your bed...