Five trips from ILL, to CA, and back taught me this...
1.) Before you leave put fresh batteries in two flashlights, charge your cellphone and shut it off, get state maps of the less populated states, ice chest full of ice only, extra water for you and the car, two qts. of oil, one can of brake fluid, one bag of small tools, a small roll of wire, roll of duct tape, make sure you have a jack that works and a good spare tire, credit card and a tow card, and if you are driving at night in those western states don't let the gas guage go below 1/2 full. Some stations have a early closing time. Some extra food that you plan on using only for emergencies. This is a list of a few things I've found helpful.
LARRY