Another thing to consider, its not all "front wind resistance" that will affect the MPG or power to pull factor. Its also the flat back of the typical TT or RV that will create a pull suction as the wind passes around it, thus your "dragging" the air with you. Watch a wind tunnel demo video and you will see what I mean.
This is the beauty of the TD shape and design, round front and sloped off rear = the best design to tow!