I don't think I'd sweat the 33 foot, the long wheel helps a great deal to prevent the tail from wagging the dog. Also keep in mind for just rated the same drivetrain (engine and trans) to 17K lbs on the new f-150 and the rear ring set on the expedition is 9.75 just like the f-150s. I've personally towed at 20K combined without issues but that was on a EL 4x4 with a 18" flat bed trailer and the weight was just machine bucket so there was no wind resistance or extreme length.