we traveled with Lilo at this age with no problem. Just make sure that you stop often for bathroom brakes, BC we did have 2 accidents in the car
One time she peed in her car seat and second time she did it on my lap.
Also wee had problem with her not eating the whole time she was in the car, but she did drink water.
Make sure you get some of her favorite food with you or some trats.
Except that everything else was fine. She slept most of the time.
You can checkout the video of her taken in the car on our way down to florida
http://www.youtube.com/watch?v=5Vkl6dv67BU
Good luck with your trip.