above and beyond what has already been listed;
Fire extinguisher, get a BIG one. Two big ones are better. Keep on in your truck and one in the trailer. If you have a wheel bearing lock up and catch fire it can take a lot to put them out, even more so if the tire catches on fire too.
a good flashlight that works. Again bigger is better. Bonus for the adjustable kind that can do a flood or spot type light.
extra fly spray
trash bags (get the super heavy industrial types) Can also double as a rain poncho in a pinch
wheel chocks in case you need to unhook the trailer
extra bulbs for the tail lights and any tools needed to replace them (and the know-how)
extra chicago screws (if your tack has them)
For long distances I carry 2 spare tires since I have had 2 flats at once one time thanks to a chunk of steel that fell off a truck in front of me.