When I moved back to Kentucky from Washington it was 4 days of driving, averaging about 700 miles a day. I did it with an SUV full of stuff, a small dog and a cat. What I suggest is put out their food and water the night before, and then as soon as you wake up the day you're leaving, take all their food and water away and leave them and the litterbox sitting out. Get the car packed up, get yourself ready, and do the cats last so they have time to use the bathroom if they need to. Don't give them any food or water throughout the day. As long as your car has A/C they will be just fine. When I would stop and get a hotel I'd put out their food and water, then in the morning I'd take it, take the dog out for a walk and make sure they both used the bathroom before I got in the car, and I had absolutely zero problems, even though I was doing 700+ miles per day pretty much non-stop except for getting gas and a quick bite to eat every 300 or so miles. If they can deal with it, I suggest fastening their leashes to something so they can't move around very much. I tied a string around the back of the front seat and tied it through my cat's collar so he couldn't get up and walk around. He just laid there the whole day, but I was close enough to reach over and pet him occasionally and give him attention so he didn't feel like he was being punished. I tried letting him run loose but after about an hour I had to tie him up because he was running around the back, then tried to wedge himself between me and the door and sit on my shoulder, and I realized he might cause me to have an accident. Cats, even the emo ones like mine, are playful and curious, especially in new environments, so I highly recommend finding some way to secure them so they can't get in the way of your driving.