Last Febuary, I drove from northern Colorado to San Diego with my two babies. I've told the story several times on this board, but I always remember different details, so I'll say stuff again.
I only had them in their carriers for the first couple minutes. I was starting out in a *very* empty part of the of the country, and even had I gone off the road, crashing was... unlikely. Besides, I had small carriers by necessity, and they would have hurt themselves if I hadn't let them out. They roamed freely during most of the trip, except during our... three? rest stops, and they were leashed during the other two times I stopped for gas and my passenger didn't feel like getting out. Not that they had any interest in going outside the car.
Rowan spent most of the time in the litterbox. They have this thing, where they'll hang out in the others' carrier, but not their own. Bella stayed in Rowan's carrier, and hung out on my passenger's lap a lot. And I have a freakin' adorable pic of her laying around my stick shift. They spent just a little time together in Rowan's carrier, and Bella slept behind my heels for a while. There was one point in the middle of Utah where she got interested in my toes, but I had their harnesses on, so it was an easy extraction. Mainly she didn't like the car noises and altitude changes.
I had their harnesses on for easy leashing and restraining, and because their identification was on there, in case they got out somehow. I also had their rabies tags on the harnesses for if I got stopped at any of the state ports of entry. And vet paperwork handy. I didn't need it, but I hear that the ports of entry are occasionally used for something other than state trooper doughnut trades.
I had water, which mostly just got spilled, and food that Rowan ate a bit, and I believe he used his litterbox at some point. Probably when we were eating lunch, or I would have noticed, I'd think. Bella was too upset to eat, and I didn't notice her drink until we arrived that night. For eating lunch, I parked in the shade, and left the windows open just a crack. It wasn't cold or hot that day, and I ate quick. If the weather had been less neutral, I would have just eaten while driving.
I drove straight through (23 hours), because I was too poor to afford a hotel room, and because it's easier for me to drive an exhausting day than to drive two half-exhausting days. I'd made the drive twice before, without a passenger and cats, so I knew it was do-able in bad circumstances.
Not having a passenger would have been awful, I think. He kept Bella occupied and off the windshield, which was distracting through Vegas. I wouldn't recommend travel with free-roaming cats and no passenger.
So... passenger, harnesses, litterbox, food/water... that's pretty much it. I probably should have had their favorite pet bed out and waiting for them in the room that night, but it was too packed in for me to care. We made do with the litterbox and carriers that night.
And, of course, cats do best in their new house if you introduce them to it one room at a time. Mine were overly-brave kittens at the three story I just moved out of, but the tiny little apartment we just moved into last week scared the dickens out of them. Keeping them in the bedroom for a couple days gave them a 'safe room' for when they're frightened by a new sound (oh so common with the construction three feet away). I was actually blown away at how quickly they wanted to start going out on walks here, with all the scary strays that the oldies around here feed.