When the car is pulling the trailer, it's not just the air that is pushing up on the car. The trailer itself is also pushing the car, especially when it's going up a hill. The trailer is also pushing the car from behind when the car is coming to a stop.
In addition to the drag created by the trailer, there is also the friction between the car's tires and the road. This friction is what allows the car to move, but it also uses up energy. The amount of friction depends on the weight of the car and trailer, as well as the surface of the road.
All of these factors combine to make the car use more petrol when it is pulling a trailer. The increased drag, weight, and friction all require the engine to work harder, which in turn uses up more fuel.
