That's a good point concerning the Nascar racers. My guess is that they draft so close to one another that they basically form one body moving through the air. As such, they only leave one wake and it only pulls back on the rear car. If so, then each car helps the other. The Winnebago is a completely different size and shape than the car dragging on it, but there still might be a similar effect if the rear car is right up tight on the Winnebago's bumper.