In addition to wind drag, the additional weight contributes to the acceleration work required by the engine. This acceleration is occurring during cruise speeds

In addition to wind drag, the additional weight contributes to the acceleration work required by the engine. This acceleration is occurring during cruise speeds as well as starting from rest (you can tell because your car engine is louder during cruise speeds than it is while the car is idling from rest.) Anything towed or carried contributes to the weight of the car which means the car is accelerating more weight. The curve isn’t perfectly linear with additional weight, as you noted, but the reduced MPG is measurable with a number of passengers in the car, let alone a towed car. A reduction of only 10-25% mileage might not be noticeable to a less observant driver but that isn’t to say it’s effectively the same.