If I had to move cross-country with my cats I'd seriously consider renting a motorhome. They hate the car, but I think the'd get used to the RV, especially when they realized that they weren't going to the vet. One issue would be that I'd have to come up with a system to make sure that they couldn't make a break for it when a human was exiting the vehicle.
Edit: not my cats, just a random image on the web.
