BossHogg and jejb are right on the money. A limited longhorn is going to have one of the lowest payloads in a 2500 already, and adding a 6.7 decreases payload by another 900 lbs. Your door sticker is the answer for the payload number on your specific truck. My truck is a Laramie and payload is only 1,681 lbs before passengers. All in I sit around 1,100-1,200 lbs available payload most of the time.
You can sometimes go a little over on payload if you can stay under the rear axle weight rating but you'll need to scale the truck to really know that that number is. A 2500 just isn't really meant to tow much of a fifth wheel. There are some out there that it can tow but to stay within your weights you'll be pretty limited in what you can choose.
My general rule is: 2500 for *most* travel trailers(some still need a 3500) or goosenecks (where you can move the load back to alleviate pin weight) a 3500 single rear wheel for fifth wheel trailers under 36' and 14k lbs, a 3500 DRW for fifth wheel trailers over 36' and 14k lbs. Makes for a much more pleasant towing experience.