My take on tools...
Select tools to perform common repairs of parts that fail due to use and not due to lack of preventive maintenance. If the latter, you'd might as well pack your shop in a trailer and pull it behind you
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The airlines rebuild/replace 90% of all parts on an airplane in accordance with a strict, FAA approved, preventive maintenance schedule (PMS, yes, that's the abbreviation). Bad things happen if you wait for a part to fail. While our trucks certainly will not fall out of the sky, getting stranded far off the beaten track due to lack of preventive maintenance can quickly become a life threatening event depending on location and time of year.
First step determining a tool kit IMO, look over the vehicle manufacturer's maintenance schedule published in the owner's manual. Depending on the part/fluid, I will often perform preventive maintenance many, many miles sooner as a normal course of action. The published schedule is often determined by analysis to determine mean time between failure (MTBF). If Murphy is with you way out in the boonies, you will likely end up with a part failing far short of the mean. Why I decrease the recommended service mileage for parts that will stop me in my tracks.
While an expensive option, I'd considering replacing 2yo+ truck crank batteries before heading on a long backcountry trip or one far away from home.
I know guys who pack extra axle bearings, ring/pinion gears, clutch/psi plate, darn near everything to rebuild drivetrain and suspension. While certainly a good idea, I guess, I would advocate replacing those parts based on miles or calibrated wear before heading out on a trip to the boonies.
As others have posted, there a far more likely parts that will fail on your rig. I would build my tool kit around those parts. Flat tires, lose of fluids, etc. I carry
Black Cat truck tire repair items,
ExtremeAire Magnum air compressor (I have it in the big honking red box, someday, I might attach the compressor to the underside of the truck frame). JB Weld (cold weld) to fix a hole in the radiator causing fluid loss. Gorilla Tape and 3 or 4 different sizes of
Thomas & Betts cable ties (a few with the stainless steel locking device). Select tools stowed in a Blue Ridge Overland Tool Bag. The bag holds a lot of tools and they do not rattle around. Further, the bag can be made to conform to a tight space, if enough room in the space.
Finally, the MOST IMPORTANT tool is already carried in the best case imaginable, your brain inside your head. If something bad (mechanical or otherwise) happens to you out in the boonies, KEEP YOUR WITS! Rather then jumping out and immediately performing maintenance, assess the situation. Rather then immediately assuming you know the problem, step away from your rig and ask "OK, what problem(s) do I have? Slow down and take time to assess the overall situation. Other then getting the vehicle safely stopped after suffering a mechanical, very, very few break downs require immediate action on your part The military and airlines are infamous for instilling this concept.
And the second most important tool IMO, water. Not just enough water for the number of days you planned to be out in the boonies, but a good reserve of water and (not or) a filtration system to purify water) for unexpected added days in the boonies. You can go a long time without food, not so for water.
And I always have my InReach with me on outings. You can not rely on cell phone service and coverage.
Sorry in advance if I've rambled on far too long. But, an important topic that goes well beyond actual tools.
