You know, I have a tool kit, and I tried to think of the three tools I use the most often. Actually, they are very simple.
1. Square drive screw driver. My manufacturer used square drive screws on almost everything they installed. There's very few times I'll go on a trip where I'm not finding a loose screw or two that needs to be tightened. This is the most common tool I use.
2. I don't know how you can do any trouble-shooting without a good basic DVM. 3. Slip lock pliers
4. Basic socket set.
5. Torque wrench for my wheel lug nuts.
6. Hammer
7. Nice assortment of screws, nuts, bolts, washers, lock washers, hose clamps, duct tape, wire, electrical connectors, and zip ties.