BoatUS membership is cheap insurance for a long tow. They will typically cover sending roadside assistance out to help, and then a necessary tow for both the vehicle AND trailer back to the nearest repair facility. I want to say a yearly membership is around $400.
Have a good spare tire and the appropriate tools to change it. Including jack and lug wrench.
I usually take a small tool bag with me as well. Small supplies to fix electrical issues (tape, strippers, cutters, splices, few feet of spare wire), pair of pliers, couple screw drivers, hammer, cheap pocket knife, length of spare rope. Get a cheap head-light (for your human head, not the truck) from Wal-Mart and put new batteries in it. This will save your butt if you have a flat/issue in the dark/rain. Cheap pair of work gloves, or spare latex gloves to keep your hands somewhat clean. Fast Orange soap, and a roll of paper towels works wonders with a water bottle after a roadside repair. Oh yea, roll of duct tape in your color of choice. This can fix SOOO many things for the remainder of the trip. It's cheap and easy to toss in the tool bag.
You can get MOST of this stuff from harbor freight for not much money. Ideally, it only has to work once or twice to get you out of immediate trouble, so it doesn't have to be the highest quality, or nicest looking stuff. Just has to work in the moment.
I've spent years both trailing fast cars around, and driving fast cars around to various car shows/events all over the country. Often times we would have just put them back together the night before, to drive them 8+hrs the next day so we could drag race them, and then attempt to drive them home on Monday. I've developed a certain "MacGyver" style tool bag that I can toss in the truck and take off and know that I have "most" of anything I would need to get me to at least the next town/truck stop.
Speaking of truck stops, there are often diesel repair shops nearby the really large truck refueling gas stations(like Pilot rest stops). If you find you're on the road and have lost a bolt, or need a special tool, those places are a godsend. They usually have all kinds of tools, and a bolt inventory sitting around. Might not be the "right" fix, but it'll usually get you home until you can get the proper fix.