Nesting trays, i like fiberglass as they clean up nice, hold hot oil, dont wrinkle.
If you use pakrat tool boxes, weld a locking latch/clasp on it, rough roads will bounce the spring loaded pin loose, viola the draw slides open.
Get your bolts organized. Look up durham steel products, or similar.
Sliding trays with tote bins for electrical, screws, airbrake, zip ties, yada yada assortments.
Spare vehicle parts... f450 and f550, dodge 4500s etc you need some lug studs and lug nuts, preferably enough for one wheel end. Fuel filter. Xtra oil. Fords... extra alternator. Freightliner service truck i had was hard on batteries, the kenworth was fantastic other than the yellow crap under the hood. Carry a jack big enough to lift the truck, and actually small enough to get under the vehicle.
Oil samples.
Safety stuff... ears, eyes, hands, lockouts, etc easily at hand, and some extras. Get an rn or emt to make up your first aid kit. Lots of gauze, tape, vet wrap, maxipads plus some of the regular stuff.
And burn free ointment. Its good stuff.
Info. Dont be cheap, get on bookfinder.com or ebay or amazon and BUY the books, manuals, specifications, microfiche etc to know your equipment. And dont be cheap on the special tooling. Its special for a reason, and that is it saves time/fingers/warranty work. And sometimes, if you dont have the tools, dont take the job on.
Use a tool tray like what is used in a graymills solvent tank, keeps tools readily accessible and dirt falls through. And alot of the daily tools fit when on an extended project, making cleanup faster.