The dealer probably thinks you'll ask for a slider hitch as part of the deal. Since he doesn't want to have to give a dollar more in concessions than is necessary, he's trying to deter you from asking for a slider hitch.
We also have a slider hitch in our 2004 Silverado and have never had to use it with our older Cougar fiver. It does ease the mind that we have it if we need it but we've never needed it. I suppose you could tell the dealer that to make this sale you want to first start with a non-slider hitch installed and then hook up to your new fifth wheel and see how much you can jack-knife the fiver before making contact with the truck. If the clearance is too little, then the deal is the dealer put's in a slider. The hitch rails will accommodate either a fixed or slider(at least on the Reese and other's like it).