Travelers generally think to bring bottled drinking water, but few realize how frustrating it is to need a larger quantity of water with no way to get it.
Some rest areas have poorly designed sinks that automatically squirt liquid soap, run water for a few seconds, and then immediately turn on a hand dryer. Brushing your teeth in these contraptions is nearly impossible.
Keep a gallon of extra water tucked away in the trunk for rinsing sticky stuff off your hands, splashing on your face, using to brush your teeth, or even filling an overheated radiator.