An abrupt change in dog food can cause, well, things you really don't want to deal with while traveling. Familiar chow and a gallon or two of tap water from home will help prevent tummy upset while you're on the move. If you feed a hard-to-find boutique brand, bring a trip-long supply to avoid wasting time chasing it down at your destination.
Tip: To avoid upchuck, feed your dog in the evening, not in the morning before getting in the car. Likewise, water him at the beginning of a rest break, not the end. For pets susceptible to motion sickness, products containing ginger can help.