I got lucky and my truck came with a good amount of stuff in the tool boxes (they had padlocks on them).
I can't believe that no one has mentioned duct tape. Don't skimp on quality. I also like to carry electrical tape in case I find a wire that had been rubbing.
Can't stress enough the importance of a good fire extinguisher... or 5. It can be the difference between having to do some electrical work and having to find another truck.
I like having a good tow strap too, Suprman has some nice 44,000 lb rated ones in the classifieds.
I love LED lights and would have a full set on my truck if the prices were more affordable. Speaking of light, a good flashlight or other work light. I'm going to mount some LED work lights on my trailer and I've been thinking of mounting a couple on some type of removable mount with a 20ft or so cord so I can use them around the truck and trailer if I have any issues. This is a gooseneck with my civvy truck, but it's got me thinking about making a plug in set that can use the slave connection for the 5 ton.