The boonie box is a good item to have on a trip, and is reasonably priced. One thing I would definitely recommend is a spare serpentine belt. It's one of the easiest roadside fixes you can do (20-30 minutes), and when one goes it pretty much shuts you down on the spot when you lose rotation to the power steering and water pumps in addition to the power brakes. Shortly after I bought my truck I bought a couple of 3" sink drain test plugs from Lowes and used them to plug the frame tube that sits just forward of the rear wheels. I rolled up some basic tools (ratchet, extensions, 1 1/8" socket for the fuel filter canister, screw drivers, pliers, etc) in a couple of rags and stuffed the bindle into the frame tubes. When the plugs are tightened the rubber center expands and seals the frame tube from the elements, so water and dirt never get inside. I've used all the space under the back seat, plus I don't like the idea of getting grease on the inside of the truck. One other thing you might do is check to see, and make a note of all the Cummins parts/service centers that will be near you on the trip. They carry pretty much everything related to the engine, and will also do engine repair work on any vehicle with a Cummins engine.