Get the bearings on the trailer checked and repacked before you leave. Test brakes. Get the oil changed in the truck, plus any other service it needs. Carry a tire pressure gauge, and use it--check every tire every morning before heading out--including your spares. Add air to any tire that's down 2 psi or more. Have any tire that loses more than 5 psi seen by a shop, or pull it off and run on the spare. Besides the normal spare (mounted, inflated, ready to go), I carry an extra tire for the trailer (and now one for the truck as well) on long trips. Test your jack at home and make sure it has adequate lift for the trailer. Make sure you have lug wrenches to fit both truck and trailer on board. Pack both where you can get to them without unloading half your gear on the side of the highway.