Here is what I use ^^^^^^^^^^^^^^.
I carry 2 of them. One is complete "fluid" replacement (oil, filter, trans oil, gear oil, power steering oil, brake fluid) and belts and hoses , hose clamps and various lengths and sizes of fuel line and vacuum line.
The 2nd box is my "recovery" box, it has my Viair compressor, peice of 2x4, HD gloves, snatch strap, tow strap, jumper cables and various other things, including a short shovel, bolt cutters and pry bar and HD hammer.
I also carry a "Survival" backpack, its setup for 3 days (food , water, shelter, fire starter, first aid, etc)
I also carry 2.5 gal of water inside the truck and (2) 5 gal gas cans on the outside, spare is checked as well.
If im going on a big adventure, ill carry a box that has my "electrical" stuff in it, so I could repair any electrical issues, it has fuses, bulbs, wire, connectors and nuts/bolts.
I carry a hi-lift jack and a long shovel. I also keep an extra change of clothes (usually a jump suit), long rubber boots, extra shoes/boots for hiking, and a thick moving pad/blanket. A couple tarps, complete tool kit, two adjustable camping poles (use for a make shift tarp shelter).
I think thats it, lol.