toilet paper, toilet paper, toilet paper. :)
oil for the engine, and a spare oil filter or two.
spare v-belts and tools to change them.
spare windshield wiper blades.
spare bulbs for headlights and brake lights and such. also interior lights.
   and several flashlights with spare batteries.
BIG MUTHA towing strap (like rated at 10,000 lbs).
paper towels and windshield cleaning stuff (like Windex).
$10 worth of quarters.
a GOOD tire gauge, and a GOOD bicycle stand-up air pump. if you get stuck
  off the pavement, you can let the air out of the tires (down to 10-15 psi)
  and it'll help you get UNstuck. but you'll need to pump the tires back up
  to drive on the pavement. yeah, it's slow with a pump, but it works.
a GOOD raincoat or poncho. preferably one of VISIBLE colors. or a box of
  very large garbage bags, which can do double-duty as a poncho (you'll have
  to make the holes for your arms and head). :)
a decent first aid kit, with bunches of bandaids and antiseptic ointment. and
  good scissors. and a magnifying glass and tweezers (for pulling out cactus
  needles and splinters from the firewood).
a good pair of toenail clippers.
if you have a water-cooled bus, carry a gallon of coolant along. just in case.
  it helps to ward off evil spirits. :)
*check the spare tire before you go*
*check the battery water level before you go*

a good sewing kit. with pins AND needles. you just never know.

:)