In an earlier thread, Bill H jokingly talked about installing a Marmon Wasp Tail on a Class C.

Here is one on a car looking for better mileage.

Boat-Tailed Car Boosts Mileage By 15%

Looks more like someone put cardboard on the back of their car. I guess you pull the tape off to get in the trunk.

Real life. Many moons ago we had a Sportscoach powered by a 440 Dodge. Nothing I did to it would increase the MPG except towing our 16' speed boat. This would increase MPG .5-.9 MPG every time. The nose of the boat was stuck in the vacuum pocket on the rear of the motor home we we really had a boat tail.

When I went from an older barrel-backed boat to one with a squared-off stern, I noticed a slight increase in fuel consumption. I know this is valid because my primary tank always ran empty at about the same spot (on the way to the River). When I changed boats, it always ran empty at the same new spot, a few miles earlier.

I always towed with a cover. Later, I installed a full-width swim step, and found that the extra length and the streamlining effect of the extended cover (which went back to cover the droop-snooted jet) let me go a little farther on the same tank.

This was a serendipitous Marmon Wasp effect, and I wondered what purpose-built streamlining appendage would do. I considered building a tapered fairing on the back or (even) loading the boat on the trailer backwards to really maximize the boat tail effect.

Once, with a violent storm rapidly approaching across a lake, we really did load a boat backwards in our haste, but never towed with it that way.

Funny thing. The Marmon was mentioned in the context of raising the rear overhang...........