Chevy has two truck cabs with 4 doors and a short bed.
A extended cab with a 6' 6" bed with a 9400 lb GN or 5th wheel tow rating. Payloads run from 1400-1800 depending on cab selection.
The other four door is the crew cab with a 5' 9" bed. Chevy doesn't have a GN or 5th wheel tow rating for the 1500 crew cab.
My wife has a '06 1500 crew cab chevy that doesn't recommend a GN or 5th wheel trailer.
I contacted chevy truck for the reason why.
After going through three female tech persons that had no idea what I was asking I finally got a line with person in a chassis engineering dept. His comment was chevy felt the 69" short bed would have too many clearance issues. He said the truck had no problems carrying weight or pulling a GN or 5th wheel trailer weight.
Thats probably the reason the OP's dealer wanted big bucks to rig up a hitch or pin box for the 4 door pickup. The devil is always in the details.
I added a GN plate to the floor of the bed of the wifes 1500 crew cab chevy so I can use my smaller 14k GN trailer.
I would contact some of the hitch makers for their input on a 5th wheel hitch for a 1500 crew cab chevy. They may not make one.
He will have to do his own searching though each RV manufacturer website and look for a 5er with 900-1100 lb dry pin weights that a 1500 truck can handle. Use this handy web page for lots of searching
http://changingears.com/rv-sec-manufacturers-type-fw.shtmlSure wish the poster that screwed up the page size would delete his post or make it a clickie.