The key for me on these long drives is to pull over and take a catnap. On the interstates, there are generally rest areas. It's not so much the lack of sleep, it's that my eyes need a rest from looking at the road.
Here is my longest example: On July 4th weekend in 2008, Mt Rogers in VA was the only state HP I hadn't done where it wasn't expected to rain. From NJ, I left the night before for the hike, probably before 11pm. Although I had plenty of caffeine, I needed to nap twice on the way - and I arrived at Grayson Highlands State Park in VA sometime before 9 am.
I hiked the mountain - no problem there - and then I headed back to NJ sometime in the afternoon. I was still driving as it turned dark again, and eventually I needed to sleep again. Since I was at a point where there were no rest areas, I got off at an exit, parked my car on a main street in a town, put my seat back, and fell asleep for a couple of hours.
I remember that when I got back to my place in NJ, that it had been about 24 hours since I had left previously.
My suggestion is: don't try to do it all at one time using stimulants. Take a nap every few hours.