Geese trade off the lead position because drafting is easier than breaking the wind, so to speak. (That reminds me of the movie "Spinal Tap." They were getting ready to come out with an album called "Break Like the Wind," as I recall.)

As I recall from engineering school (over 30 years ago!), the equations describing the aerodynamic drag on the Winnebego are parabolic equations, which means that the only conditions you need to know are the upstream conditions. What ever is happening behind the Winnebago has no bearing on the drag it sees. As I recall, the story changes as you cross the speed of sound. Then you get into hyperbolic equations that require you to know the downstream conditions as well.

Or something like that.

Your friend raises an interesting puzzler, though. On the surface, it seems like the guy dragging off the Winnebago is getting a free lunch, which generally doesn't happen in nature.