The bigger question is once you put the 5th wheel trailer on there, (assuming it's a camper), will the toolbox be hitting the camper when you turn??? I had a small toolbox in mine and the camper cleared it ON FLAT GROUND by about an inch in a hard turn. If you had any kind of hill or dip it would hit.
Text me at 919 605 6040 and I'll take a picture and send it to you and get you whatever measurement you need. It'a a short box Chevy.