My cat survived many trips in our daily driver from Los Angeles to Mammoth Lakes, about 6 hours drive. He had his carrier with his quilt and the door open. We stopped three or four times for his water and potty breaks. He would crawl into his carrier and sit or snooze or roam around but he was not allowed on the floor. In the MoHo he slept most of the time either under my duvee or in an empty cabinet under my bed with his quilt.
He figured out if we were in the MoHo or in the loaded car that we were not going to the vet and never cried, in the car without luggage ect, howled like crazy.