A couple of things that seemed to help on our trip last to WDW a year ago....
First, we had everything except the barest of essentials packed in the van the night before... we then got to bed SUPER early-- like 6-7:00 p.m.... even with all the excitement (it was a WDW first for ALL of us!) we managed to get a few good hours of shut-eye....
I set the clock for 2 a.m. This "trick" goes back to my college-finals days... I used to do this very thing when I needed some heavy-duty studying. I always found that after the initial "shock" of waking up at that awful hour, I felt great and more alert than any other time.... probably wouldn't work for everyone, though.
We quietly and quickly moved dd6 to her carseat without waking her, and gathered the little bit of stuff we still needed... I was already fully awake/ showered, etc., so I did the first pretty long stint of driving (4-5 hours,) while dh napped-- he is a much better "car-napper" than I am.
Our dd didn't wake up until around 7:30, when we stopped for breakfast/ potty. That was about 5 1/2 hours we didn't have to be asked, "Are we there yet?!"
Since dh had practically a whole night's sleep, he was good-to-go for the better part of the day of driving. We did stop at a motel for a few hours when we were both tired.... really, our opinion was, what's the point of getting to WDW totally exhausted and cranky, anyway?
I agree with the poster that said the way home is much different... we left WDW with a full-night's sleep, but I was totally useless, as a driver... thank goodness dh wasn't as tired!!! We started driving around 8:00 a.m. and stopped about 5:00 and spent the night. I felt like an absolute zombie!!! I couldn't have driven even 1/2 hour that day.
But we did the 2 a.m trick again after stopping for the night, and felt great.
HS