We're only planning on driving 6-10 hours a day, so we aren't going to stop for extended periods of time. During gas and pee breaks we'll open up the large hatch in the back of the truck to let some fresh air in, but we aren't planning on stopping for lunch or letting the chickens out. The only real stopping that we're going to do is when we park for the afternoon/night. With the trailer attached to the back of the truck, there also isn't enough clearance to actually open the door, so we're going to have to open the hatch and crawl over it. This means that the truck bed will never really be open. I'll just close it behind me and only open it again to get out and back in the next morning. I'll have to put all of the chickens back in the dog crates, but we've moved them like this before and they aren't too difficult to corall!
We're going the northern route across the country, so luckily it won't get too hot. We're also in the rainy season, so we're looking at 50-70 degrees partly sunny or raining most of the time. Truck beds have a lot more ventilation than most cars do. The last time we moved we had the cats in a large dog crate in a similar setup and it never got above 70. It's actually cooler in the back of the truck bed than it is in the front.
I'm also really not worried about predators. The windows in the back of a truck bed don't open like normal windows do. They just pop out slightly, about 1/2 of an inch at the largest point. Unless you picked up a gardener snake and dropped it through the side of the window almost 5 feet off the ground, no predators are getting inside!
We could make them a little pen inside of the trailer when we stop for the night, though. We also have a collapsible chicken run that we're bringing. We could put them in the run while we're outside and can watch over them. I'd be a little worried about them picking up strange bird diseases while on the road, but it's a possibility. Especially if it gets hot while we're stopped.
As for food and water, I'm going to put a dish in each of the dog crates while on the road so they'll have 24/7 access to food. I'm not entirely sure about what to do with water, though. Everything that I'm imagining will result in some very wet chickens! When we stop for gas I could fill up their water dish, let them drink, and then take it out again. Obviously when we stopped for the night they'd have complete access to food and water. We aren't planning on being on the road too much every day, so this might be fine. We could put some particularly hydrating foods in with them while on the road, too, like a watermelon.
Thanks for all of the advice everyone! I'm starting to feel more confident about this. I'd really rather take them with us than try to ship them halfway across the country or leave them behind.
It might not be their favorite thing in the world, but I'm trying to make it as comfortable as possible! I'd put them in the trailer but it's against the law to have live animals in a trailer while on the road. They also wouldn't fit in the front of the truck (we only have 1 row of seats).