After much "long trip trial and error" here is what I have learned! Don't over feed and over water. They don't need more than they would normally consume at home. They also don't need to potty more often than they do at home. I'd stop every 3 - 4 hours for an adult dog. What is different, though, is the activity level. During those potty stops, schedule enough time to be able to stop for 20-30 minutes. Let the dog walk all over and sniff around, or just walk some and sit in the sun some. Play with him, throw a ball - whatever - just let the dog move around more or less at will during that time so he won't be restless in the car. One suggestion - take a tie out so you can all picnic together for meals. That's a great way for the dog to be outside, walking around when he wants to and lying down when he wants to - then afterward, you can walk him around and play with him harder, then he'll be good to go for a while longer.
Good luck and enjoy your trip!