Remember that the birds will be stressed so everytime you open the cage you have them in you take a chance on someone escaping so it might not be wise to remove them and put them in the trailer. A large cage in the back of the truck would definately be your best option. It should be covered to give shade and protection if it rains. You can cut out a gallon milk jug, like from about half way on the front and up, to make a waterer and mount it by the handle on the side of the cage. Then when you stop add small amounts of water so they can drink till everyone's full but not leave enough in it to spill when you're driving. And you could put their hanging feeder, if that's what you use, or a large dog bowl of pellets (which should be heavy enough not to blow out) for them to munch on. The watermelon is a good idea, but the more of it they eat the more watery their poop will be. I brought 5 standards 7 hours when I moved here. They were in a very large wire dog crate with a bowl I used to water occasionally. I also mounted a roost in there and that's where they spent most of the trip. They'd hunker down on the roost when we were driving then when we'd stop they would get down and drink. I hope this helps and I hope you have a good trip.
