adding a leaf spring

Have you weighed your TT? Dry weight is around 7200lbs. That's 8200lbs loaded for the average camper. TW will be near 1000lbs when loaded. My guess is you don't have enough truck. Ford F150's are soft in the rear and sag easily. You would need 1400lb spring bars to return the front to 50% or more of the original height. Even then with the initial rear sag it's tough to get the front back. BTDT. Have you thought about Timbrens? I have them on my 2500 and unloaded they are non existent. Rides the same. They sit 1" above the rear axle. When you drop the TT coupler on the WDH the rear will only drop 1"+. Then you can hook up your spring bars to set the front of the truck. 20-30 mins to install, no air leaks, ride is same as stock and the rear won't sit higher when empty.