Lots of people end up with spacers under campers for lots of reasons. Many of them actually bolt the spacer right to the bottom of the camper so it's always there, but you could just as easily have it loose. You'd want to bridge the bed and tailgate so the back of the camper is still supported, assuming you put a longer camper in your shortbed. (I highly recommend an 8' camper for the added room it provides!)
You don't have to cover the whole floor, so even just some strips of 1x6 or something like that would work fine, so long as they left space for the hitch rails to sit in a hollow.
The alternative is to get ride of the 5th wheel rails in the bed and go to a B&W gooseneck hitch with a 5th wheel adapter. I think B&W calls theirs "Companion", but there are several other brands that work just as well, and leave no rails in the truck bed. I had rails welded to some 1/8" sheet steel with some angle iron ribs for gussets and a square post sticking out the bottom that plugged into my B&W for a while, but I gave it to one of my friends. It worked awesome, and allowed me to clip a standard rail Reese or Drawtite 5th wheel into my truck.
Good luck!
Chris
