The most efficient way to move cargo depends on its shape and dimensions as well as its weight, but generally speaking we would expect a light truck to be more economical than pick-up and trailer. Although the truck may be thirstier on its own, the fact that you have all the cargo inside the vehicle means its aerodynamics are unaffected. With the pick up, the extra weight at the rear and the increased aerodynamic drag will mean a bigger drop in fuel economy.