I keep a small duffle with some clean clothes in each truck I'm driving. I also have a mil-spec grenade box that I keep my assortment of commonly used tools for my trucks (used the box cuz it's somewhat shallow and long enough for socket rails, plus waterproof) and a SAW ammo can with a 10' strap, 30' strap, large and small shackles, 2' length of 7/16" hardened chain with a couple different hooks for the ends, and an extra ratchet strap. Also have a fiberglass D-handled spade shovel and a hammer, plus two MRE's, couple bottles of water and a small bottle of alcohol (usually rum or vodka). And a first aid kit. Have a couple packs of those snack crackers in the glovebox, couple pairs of disposable gloves, sturdy fixed blade knife (of adequate size for gutting a deer), small handful of zip ties, and a compass. Under the seat there's a roll of TP in a ziploc bag, roll of duct tape, one of those thin divided boxes with some assorted fastners, bulbs and fuses, and some WD-40 or the like. Usually have a lighter and pack of matches on hand as well and a couple maps, and a camera goes nearly everywhere I go.
My work truck carries all that and enough tools to build a resort....