You can special order a Chevy 6.6l 4x4 DRW "work truck" built to tow a fifth wheel (pucks in the bed with spray in bedliner) with "basic conveniences" like cruise, upfitter switches, electric windows, remote start, etc) for an MSRP of $48K. You should be able to negotiate a price close to the asking price on that truck (and this one comes with a factory bed)...
https://www.chevrolet.com/trucks/sil...-price/summary
The "bed delete option" reduces the truck by about $1300. You won't find a "aftermarket bed" for less than around $5K, so even though that truck is under $50K, it's going to be a $55K truck by the time you put "something flat" behind the cab....