In spite of living in Alabama for twenty years I can't get over the jumper cables, space blanket, tow rope and folding shovel habit. Cell phone charger (and phone). Paper towels can substitute for almost any "tissue" product and even coffee filters. Zip ties, an assortment of fuses and light bulbs. Sealed beam headlights were too big to carry but now having a spare headlight bulb is easy. Battery terminal cleaner.
I'm guessing most of us carry a fairly complete tool kit but a couple pair of small needle nose Vise-Grips are very handy pinching off almost any hose while a field repair is made or an inline filter is changed. If you have a friend in the HVAC industry talk them out of a short roll of good aluminum plenum tape. Sticks to almost anything, broad temperature range and will seal a puncture/leak in camper/trailer siding.