M.B.
I just did the same exact trip with my 3 year old son. I sat in the back with him, and brought a plastic storage container of things like puzzles, books, flash cards, crayons, coloring books, stickers and paper, playdough, and dvd's. I made sure to alternate these pretty quickly with him so that he really never got the chance to get bored with them. I went and got a lap desk at Ikea for $10, the kind that has a bean bag on the bottom and a desk on the top and it worked great as a surface for him to play playdough on, and color on. Im not sure if this is an option for you but I also brought my laptop with some preschool age programs, and this was by far his favorite thing and kept him busy for hours on end. I got a lighter adapter for charging purposes.
But one thing I did do that I think helped a lot, was I brought a little bag with bubbles, a kite, and a beach ball, and we stopped every few hours at a rest stop and flew a kite, blew bubbles and chased them, or kicked the ball. I snuck a lot of running in there with "Hey can you beat Mommy to that tree?" and other things to let him run some of his energy off. We also stopped at a hotel that had a pool so I took him to the pool that night and let him play, in hopes that he could burn off enough energy to go to bed at a decent time that night, it seemed to work.
Its a long trip!