50ft Gladhand Airhose and a good Dial-Type Tire gauge
To keep all the tires properly inflated. Calling a Road Service to replace a tire that was under inflated and blow is expensive, including the time wasted in waiting for the Road Service guy to show up.
Checking all the tires and making sure they're equally inflated helps them to run cooler. Takes less than 15 minutes to make sure that trailer you just hooked up to is road worthy. Might save you 2 to 5 hours waiting.