Some things I've found useful on my cross country old truck adventures:
fire extinguisher

(Believe me, there are few things more embarassing than to be on the side of the Interstate with a burning firetruck ...

)
spare fuel pump and fuel filters (to go with Grigg's spare hoses and fuel line)
Rags
Flash light
Tire goo (fix a flat)
wire repair kit (electrical tape, wire, stripper, cutter, splicers)
Duct tape (The handyman's friend)
Hand cleaner
Cell phone
safety wire and a couple of bungee cords (you never know what will start flapping)
CB radio with weather channel