For me it's carrying a variety of tools that can handle different situations that I know I can fix on the road.. Too many to list that I take, so take whatever you know how to use and what you think would be handy to have...
I also carry a small floor jack, bottle jack, jack stands. Spare tire for the trailer and the truck of course.. A 12v air compressor, 120v battery charger and a battery load tester. A variety of fuses for the truck and trailer..
I carry way too much stuff that I have never needed to use.. But, in the end, I just like taking it because the last thing I want to say when I'm on the side of the road is "well ****.. I have that at home.."
Good luck!
Mitch