We bought the biggest soft crate we could fit in the back seat of our Honda Civic. With the soft crate we could collapse the top when loading and unloading then enlarge it again once the cats where in the car. The soft crate also had entry flaps on the top and sides so we could reach in and pet the cats or give them water, food, or a litter box when needed.
We read that you should take your cats on a few test drives before the big trip since most of cat's car rides end up at the Vet. This way they hopefully learn that a car ride doesn't always result in a thermometer being jammed up their butt. On the test drives we learned that letting the cats out while driving is crazy. They crawl all over the place making things pretty dangerous if they start going in the driver's direction.
Along with the cats, we put food and water in the crate as well as a cardboard Scratcher lounge that had been in our house. I think the scratcher was the most important thing we gave them. It was a comfortable and familiar thing and a place for them to let out some frustration. They tore that thing up like they've never torn up a scratcher before.
We stopped for food and gas around noon each day. We ate at the gas station for efficiency and because this is when we gave the cats an opportunity to use the litter box. As soon as we stopped the car one of us would go inside and get food and use the bathroom while the other would put litter in the litter box then put the litter box in the crate. We always kept the car doors and windows completely closed during this time because the cats could and did get out of the crate as the litter box was being lowered in. Only one of our cats used the litter box during these stops and he only peed a little bit but we felt like it was worth it to give him that opportunity and prevent him making a big mess in the crate. After he peed, we took the litter box out, scooped it out, put the litter in a bag and packed everything back away.
Each night we consulted Pets Welcome to find a hotel. At times we said our cats where a dog or that we only had one cat. Some hotels charged extra for pets, others didn't. Once in the hotel the cats where happy to eat, drink and use the litter box. After a few days on the road they even came out of hiding and curled up with us in the hotel bed.
Our cats handled things pretty well. They never fought and didn't whine as much as we expected. We did have a day with three hours of whining and yowling which was on the 3rd or 4th day. Every cat is different but this was our experience. Good luck to anyone traveling long distances with their cats. It is not the ideal way to travel but we love our cats so we sacrifice a little to make sure they are safe.