I would be concerned with that salesman !!!!
Go here:
http://www.ford.com/trucks/superduty...tions/payload/
Scroll down the left side to "F-350 SRW 4x4" then read across to the "Crew Cab 156.2" column and you'll see the payload listed for each of the "GVW ratings" usually the 6.2L gas engine trucks are rated 11,300 and the 6.7L diesel trucks are rated 11,500. so, the "normal average" for gas trucks would be a payload of about 4400 and the diesel would be about 3970.
Keep in mind that options and equipment added to the truck will change the payload, but these figures are from the Ford website Superduty specs page listed above.
Hope it helps a little...