K.J.
I think long road trips warrant flexibility in certain rules, such as no food in the car.
I'd even consider turning her seat around just for this trip. The way I look at it is that when a child is screaming in the back it is very distracting to the driver and could result in a crash.
Make sure you have something to block the sun from her eyes. Any chance that she gets motion sickness? If so you may want to pick something up for it.