While I would not say towing an RV would be a disaster, I do not think you will be happy with performance or handling when towing much more than about 4000 pounds dry. You have the smaller V8, 4-spd transmission and 13 year old suspension.
A stop at a landscape place or CAT scale would tell you the payload as your age truck was before they added the Tire and Loading stickers. Let's guess 1400 pounds. A 6000 pound TT will have about 800 pounds of loaded TW. A 6000 pound 5th wheel will be more like 1200 pounds, plus the 200 pound receiver and people in the cab.