I have used everyone of these many times and often had to track them down and buy them on the road.
Brake spoon
Couple of cans of brake cleaner
Extra bearings (packed)
Extra key for every lock
Plumbing fittings to cap any line
Extra fuses (esp. 15 & 20 amp)
Misc. crimp electrical connectors. wire nuts and crimp/strip tool
Jack and star wrench for trailer (I also carry a torque wrench)
Dry film and wet lubes
Prybar
4 way shoe rasp
12 in 1 screwdriver
Hammer
Vice grips
Box wrench set
Pliers
Needle nose pliers
Socket set with extensions
Shop towels and hand cleaner
Cotter keys (for both truck and trailer hubs)
Extra lug nuts