Long box trucks are nice for towing the fiver or when you have large loads like sheets of plywood etc. But a long box is harder to move around parking lots and other tight places.
I currently have a short box, and with my 30ft fiver I don't need a slider hitch. If I were to move a friend's fiver, I would likely need a sliding hitch. My point is that sometimes, you can get away with a short box, sometimes you need a sliding hitch. It all depends on how the hitch of the fifth wheel is configured, some project out front more than others.