Toilet Paper. As a hunter, I've learned that is one thing you don't want to be without.
Occasionally my gallon ziploc baggie with 2 rolls of toilet paper rolls out from under the back seat in my truck and everyone asks why I have it. They've obviously never been in an urgent situation where the next gas station was more than .25 mile away.
I have a complete set of camping gear either a 3 season tent or 4 season tent depending on time of year, cold weather bag, ground pad, propane bottles, propane lantern, single burner stove, propane sport cat radiant heater, 1/2 a case of vienna sausages, a case of campbells chunky easy open top soups, a case of bottled water, 2 tarps, rope, hatchet, limb saw, jumper cables, tire chains, first aid kit, space blankest, hand warmers, fire starters and various other stuff in the tool box of my truck. I need to get a pack to add to the truck in case I ever need to haul any of it with me if I have to strike out on foot for some reason.
Center console has a Fenix light 2 sets of batteries, anti histamine, advil, tylenol, lotion, lip balm, and a knife.I keep a couple of bottles of water in reach in the truck itself
I would love to throw a .22 and a bulk pack of ammo in there as well for a just in case but the issue with that is working for a school district makes it no bueno. Never needed a bit of it but going camping with my son it just became habit to leave it in there rather than load and unload it all the time. If I were to ever get stranded say because of a snow storm and run off the road I could shelter and survive for quite a while. Winter time I throw my hunting overalls, 1000 gram thinsulate boots, parka system, and heavy winter gloves in the truck as well.
I also keep a spare change of clothes in the truck, which at one time was a bone of contention between my wife and I, she had a friend that was convinced the only reason to keep a change of clothes in the truck was because I was cheating on her. Nothing nicer than having a dry pair of socks and shoes after you get soaked being a good samaritan helping someone out in inclement weather.