ATC Cougar (8 ft. long, cabover) on my F150. Started as a shell with furnace and cookstove. At that time, weight was under 1000 lbs (9xx, I don't recall exactly). I've added a bit but I'm sure it's still well under 1200 lbs. Rides fine on the F150 without any suspension modifications (but this is one of the newer F150s with aluminum body, so larger payload). I would talk to ATC and when you get one from them, try it on your truck and then decide whether you need to modify the truck.
My two cents.