The part that is going to hit is not adjustable. It is the mounting tabs or plates were the pinbox bolts go through. I thought about raising the hitch but then I will probably be noise high on the 5er.
I had the exact same problem, the main frame for the pinbox was too long.
I contacted both Forest River (Cardinal)and Lippert (pinbox mfg)and after discussing the problem was told it would be OK to cut off the trailing edge of the frame mount but to
be sure to cut it at an angle, not to square it off.be sure to cut it at an angle, not to square it off. I also looked at many other 5ers and none were left like mine, all were angled upward at the back of the frame.
I had a friend with a hand held grinder cut it off at at an angle. Took about 2" off the bottom end of the back side. I just marked where I wanted it cut with masking tape and he followed the line. Only thing to watch is to leave enough room at the top edge for the grinder to finish the cut. Took him maybe 3-4 minutes per side.
Our truck is a long box. Now I can make a 90 degree turn if need be, no more worry about it hitting the bedrail.
Worked for me, hopefully it will work for you.
Paul...