2007 f150 4x4 front end problems

i bought this truck new 2/26/07 it has 13000 miles on it. the problem is it keeps on wearing tires on the outside edge mainly. it has had 4 sets of tires on it, 2 front end alignments and 3 sets of rotors. the dealerships says nothing is wrong. can anyone guide me in the right direction?

do you carry a lot of weight with the truck. if you do it could be caused by the truck rear end saging down too low causing the front tolift and produce positive camber. get a lift kit if you do. some helper springs for the rear. or some bags that can inflate when you carry a load.