oil change.
all fluids topped off
all tires in good repair, spare checked. maybe a portable pump.
jumper cables
a quart or two of the oil
a blanket or two (how many people in car?), will you be in deserted areas?
some bottled water, some crackers, flashlights (check for batteries), portable CB radio to call truckers for help...

all of that - assuming, you will be far from civilization.
otherwise, ATM card, couple credit cards - you can buy anything on the road...