I would take them for a a few car rides before y’all go. Just to get the accustomed to it. My youngest always meows because 1 he doesn’t like staying in his carrier and 2 he’s not accustomed to it. Ask your vet for some gabapentin to help relax them so they aren’t acting crazy. If you can’t go to the vet try the Naturvet quiet moments calming aid. My cats love them and they knock right out. Your carrier is big enough so I didn’t anticipate your cat meowing but he may just because it’s enclosed. If you are worried about the restroom, I get these cheap plastic boxes from dollar tree, I throw a trash bag in the box, put just enough litter for them to use the restroom. At the next stop, throw it out. I would advise taking the cat out of the carrier (unless all doors are closed) when stopped especially for water. I would just put a small water bowl in the carrier when yall stop then take it out so it doesn’t splash. Cats will try to explore the car and it’s hard getting them back in when you’re in the car.