Yeap - that frontal area is a giant parachute just holding the air back. As Mark said above, it is the air flow but also the actual frontal area that hurts. The air flow is one of the reasons that the air wings that we see on some trucks don't really work all that well. Our trailers are too far back from the airfoil so the air just 'burbles' back behind the truck and ends up creating a great deal of drag. There is a product that was spoken about here in the forum some time ago called "airtabs" that reduced that burble and supposedly resulted in improved mileage. As memory serves, several members were going to add them to their TV and Montanas, but no one ever reported back. The web site for those is
http://www.airtab.com/en/35.html .
One other thing -- resistance squares as MPH increases -- so the faster you go, the more air you have to push out of the way and the more resistant it is to moving. Ideally we'd all be driving 45 to 50 mph, however that is not realistic -- we'd be a traffic accident just waiting to happen as someone comes up behind us!!!