While payload is often the limit for half tons, I think it will be acceleration and power for your truck.
I'm not sure any high walled RV will be acceptable to tow with that engine. It takes a lot of power to pull two sheets of plywood through the air at 65mph. You'll be cruising in 4th gear at 3000rpm, I hope. Then you hit an overpass and need 3rd or 2nd for hills. Are you ready to accept that situation? Are you prepared to upgrade the truck if you do not find performance acceptable? If you limit yourself wit this truck and get something with more capability later, you may regret the compromise with the RV you chose.
If you plan to stay below your tow rating, then under 4500 pounds should be good.