I haven't really made a kit, and don't have a lot of room, but I threw a few things in the back.
A small shovel, a hatchet, some rope and tie-down, a case of water, which gets rotated, a blanket or two, some flares and other stuff.
Should go without saying there's a light in the truck.
I take with me a sling bag, and it has more. Several days of food, usgi poncho, couple different ways to start a fire, knife or two, water filter, paracord, some other stuff I can't remember at the moment.
If I'm going more than a few miles away, the bag goes.

Oh, there's a topo map of the state under the seat.