At 7250 dry, you'll likely scale at around 8500 packed and ready to camp. The 4500 payload figure you saw is for a reg cab long box 2WD, etc. Average cargo capacity on these trucks in crew cab 4WD form is usually about 3000 lbs.
That said, you should have no issues with this truck. The 6.0L is a proven durable engine and should be good for 200K and the 6L80E trans mission is also pretty sturdy. Pull a Carfax if you can and see if there is a service history.
Any 2500 truch should do a good job pulling the new TT, now just in terms of power but handling and stopping as well.