Traveling on the same roads at the same speed and wind temperature conditions you should not see basically any difference in mpg's. The extra foot in height should be offset by the better aerodynamics of the new front cap design. I'm assuming both towables have the same tire size and inflation as a softer 65 psi inflated tire will squat more and have a larger footprint on the road's surface which relates to friction than the same width tire carrying the same weight inflated to 80 psi. A small difference but it is a difference and does alter rolling resistance numbers.
Air resistance actually relates to surface friction.
A simple basic factor to keep in mind is that at 55-60 mph towing, the average air (wind) resistance in your RV example is about 2 times (2.1) the rolling resistance. Towing the same RV unit at 65-70 mph the air (wind) resistance increases it to 3 to 1 (3.1).
The rolling resistance of a given wheeled vehicle remains basically a constant regardless of the speed you could pull it at on a road. However, the air resistance squares itself (multiplies) as the speed increases beginning at approx just under 50 mph. As in approx 2 times for 55-60 mph and 3 times for the 65-70 range. 80 mph would be cloe to 5 times.
This is why "wind tunnels labs" are used to measure actual air resistance. Aerodynamics play a huge part in altering the averages but even a very sharp needle going thru the air still has ever increasing air (wind) resistance as the velocity increases. The angular sides of the increasing needle diameter is forcing the air molecules to move away from the needle surface and thus creates friction and resistance.
Now add in the air forced compacted air behind the end of a needle or an RV being sucked back to fill the void (space) where the needle or RV had occupied and a sucking action pulling backward is the result. This resistance is friction and then sucking and at very high speeds it even creates extreme heat.
That's why the space shuttles must have extreme heat tiles on it's forward surface for reentry and even they glow bright red from the extreme heat that results. They want the sucking action created as it helps the shuttle to slow naturally but you don't want that sucking action pulling your RV unit backwards. The faster, the higher the sucking force is.
A superb CC LB 4X4, GM HD Diesel, airbags, Rancho's, lots more
Lance Legend TC 11' 4", loaded including 3400 PP generator and my deluxe 2' X 7' rear porch
29 ft Carriage Carri-lite 5'er - a specially built gem
A like new '07 Sunline Solaris 26' TT
