At higher speeds, it is actually better to keep your windows up and use the a/c. Having the windows open even a little creates a huge amount of drag. Use the lowest blower setting you're comfortable with.
Are you doing the trip in one shot? I did a 1000 mile trip in one shot...even doing 100mph for about an hour in a really, really desolate part of New Mexico (passed no cars for over half an hour, got passed once) it was around 14 hours. By the end I was fighting to stay awake.
It didn't help that one of my pit stops was around half an hour though...chose a town that made you drive a few miles away from the freeway to get to all the gas stations, fast food joints, etc.