Additional spring leaves or airbags?

My 2005 F250 had the camper package which included the rear upper overload spring and stabilizer bar. I easily had 1000 lbs of tools and parts in my bed while towing an 11k+ lb Weekend Warrior and did not see significant sag. I did however load the toy hauler so I had under 1200 lbs on the hitch, and made sure my WDH was properly set up.

Before adjusting suspension try adjusting your WDH. If your suspension needs help, start with Torklift StableLoads on your existing suspension to preload it sooner yet not affect unloaded ride.