My parents used to drive long distances at night when we were kids, it was more peaceful for them lol. Car seats and rules were different back then though, so I wasn't in one and used to just have my pillow against the window to sleep on.

We've also done a few longish trips, but I find that even with DD's seat in the semi-recline position her head still falls forward so I'm always reaching back to adjust her. I know you can get those little neck pillow things, I don't know how safe they are or if there's an age range, but maybe they're something to check out?