long bed vs short bed for towing 5th wheel RV

My short bed works great with a rail mount Andersen hitch flipped to the rear, no slider necessary. Long bed vs short bed is pretty much irrelevant from the standpoint of towing a fifth wheel. You just need to balance bed space versus parking, u-turns on tight roads, garage length and etc. Long and short beds both have various pros and cons to consider.

In a new diesel truck, a SRW will generally work just fine for the size fifth wheel you are contemplating. If you prefer a DRW that's fine too, though.