I've taken many long drives in a Vanagon.  (like 1000 miles a day for 3 days straight NYC to Seattle by myself in a stock Syncro Westy)

Satellite radio helped, but the best way to stay alert IMHO is to snack on sunflower seeds with shells on.  The amount of work it takes to shell them using your mouth keeps me alert.  I spit shells into an empty coffee cup, though many somehow end up on the floor at the end of the drive.

David brand "original" are my favorite, but my mouth gets really dry from all the salt after eating a large bag.  They also have reduced sodium, but they have reduced taste, along with different flavors.

I learned the "sunflower seed trick" from a truck driver, and I've even seen caffeinated sunflower seeds for sale at a couple of truck stops....must be popular with the pros...