Short or Long box for fifth Wheel
I have had 2 FW's, a 25' and now 29', both with extended pin boxes. I have towed these 2 with 3 short bed trucks (a F-150, F-350, & Dodge 2500) all with the same Reese 15K (non-slider) and never had any problem. If I was buying a new hitch, I would get a slider (just in case).
Usually when this question is asked, many folks write in and note that they have slider hitches and have never used them. You don't need a long box to pull an RV with an extended pin -- in fact you also don't need a slider hitch.
Get the truck that works for you overall. I use my truck about 25 days a year for RV towing and driving to work, etc. for 200+ days. I couldn't get a long bed in my garage. Good luck!