Like Fred, I've spent some time backpacking and learned the joys of a discipline of minimalism.
Since I have a front storage trunk, it's more like backpacking here in the flatlands and I don't have to practice that discipline quite as religiously. I can let my "plan for all contingencies" nature come out a bit.
For tools I keep a "101 piece" tool kit in the camper that I've had from my popup days. It has a ratcheting screw driver with bits, wrenches and a small vice grip. Most of the use has been to help other campers.
My truck jack works for the camper. I carry a breaker bar and a torque wrench with an extension and socket to fit the lug nuts.
A small tool box that has an electrical multimeter and various fuses and light bulbs. All these tools are the cheapest I could find, Walmart and Harbor Freight are your friends for these.
An Aquatainer (small or large, depending) for grey water with a garden hose piece and 1.5 inch adapter that stay in a mesh bag. In that bag also carry adapters for 30 amp to 15 amp and 50 amp outlets. The 15 amp has been used occasionally. The 50 amp is probably unnecessary.
Fresh water supplies include two 25' lengths of flat RV hose with a pressure regulator and 90 degree elbow on one of them, for city water. Then another mesh bag that has a short hose RV hose piece and a water thief for attaching to un-threaded water spigots and a spout for filling my tank from a hose. At home I have a 25' RV hose for filling the fresh water tank.
For leveling, I use a Bal leveler that also serves as a chock. For the high side tire, I have a Bal chock. These together with the stabilizers will lock the camper down so it feels as stable as a concrete slab house. I also carry two plastic chocks. The leveler is operated with a drill that I also use on the tongue jack and the stabilizers. Each is fitted for use with a 3/4" socket.
A propane stove and lantern that I've had for 20+ years with several bottles. a Coleman small folding table that can seat four close friends. A smaller aluminum X folding side table and a plastic X table that is the dish washing station. Also for dish washing, there is a scrubby and small bottle of degradable soap kept in the outside shower.
I do a great deal of hiking and geocaching, so I keep an old small day pack with two one liter water bottles in the outside storage tray, because there is room and you never know.
A small pouch with USB adapters & cables, an audio patch cord for the stereo, earbuds and a pair of cheap reading glasses.