My dad and I traveled by car from Northern Canada to the United States with my cats when I moved, one overnight on a ferry plus three full days of driving.
I did keep my cats in carriers for safety, and they were fine. They mostly slept in the car, and made up for it in the motel rooms.
The first day I tried to offer them food at lunch time, but they didn't want it, so after that I would just give them their water bowls whenever we stopped for food or gas and then dump the excess water when we set off again. I also gave them some water on my fingers from time to time during the trip (this only works when another person is driving.)
Each night I would set up the litterbox first thing when we got to the motels, but they seemed to be fine holding it during the day while we were driving, and I had no accidents - I think they were too nervous to want to go.
On the vet's recommendation I did not use any tranquilizers or sedatives. There was a little fussing from time to time but not as much as I would have expected.
They had completely bounced back within a day or two of arrival. Really, I think most cats would be okay in the car. They'll hate it at the time, but they'll get over it. Mine did.
Best of luck with your trip!