You can't make a 90 degree turn with a short box truck and a fifth wheel - period. Simple rule of physics..."two objects of mass cannot occupy the same space at the same time".
Measure the distance from the center of your king pin to the edge of your trailer - it will be 48" (more if your trailer is a wide body model). Now, measure the distance from the center of your fifth wheel hitch jaws to the back of your cab - it will be less than 48".
When I installed our Reese hitch, the manual called for the center line of the king pin to be indexed 2 5/16" forward of the centerline of the rear axle. I don't remember the measurement from the centerline of the pin to the back of the cab - but, it's way less then 48". I don't have to slide the hitch very often - but I'm glad I have it when I need it.