In July, MrsGryn and her extended family, including 2 cats, are relocating from California to Maryland; the cats will be with them on the road, all 3,000 miles of asphalt and concrete. Since I've been there and done that in my ancient past, I want to be helpful by butting in with my 2-cents worth. In the process, I thought it would make some good blog fodder, so everyone is invited to this part-tip sheet, part-travelogue.
Before there was my adorable Honey, there was equally adorable Scamp (dark fur) and Bubba (light carmel fur). I relocated from AZ to MA by auto. My dad built a cage that enclosed the entire back of my hatchback, not unlike the barrier in taxis.


One of the other relocatees at that time was a friend of mine, Johanna, who also had two cats. We decided to travel the same route and meet up in the evenings. She had a deluxe truck and also built a cage in the section where rednecks mount their gun racks.
Tip #1 - make sure the cats have all their shots up to date and travel with a copy of their papers. You don't want your darlings to be quarantined for a month in Arkansas.
Tip #2 - Get your cats outfitted in good collars and have tags with their name and a contact phone number to report to you (or to someone who can be a contact point enroute) if they do get lost and found. Pet stores have self-service tag engraving equipment. Have a leash for each and always attach it and get a grip each time their enclosure is open.
At the beginning of the trip from Arizona I "slightly sedated" my cats with 1/2 of a knock-out pill that the vet gave me. Despite this, both cats cried for a good hour (probably more) before settling down. Our cross-country trip was also in the heat of an August summer and on the 1st 2 days in the desert it was also possible that the cats were a little dehydrated and very hot. By the 2nd day I thought they were more distressed by the effects of the sedative than by the travel itself and so I stopped giving it to them. They seemed much happier, but perhaps this was just because we were in a cooler climate and they were getting used to the drill.
Tip #3 - Don't sedate the cats unless they are petrified and don't calm down within an hour or two.
As you can see from the above photos, my cats became first-class travelers after a bit. In fact, sometimes they were so quiet I had to check to ensure they were not left behind.
Tip #4 - Always make sure they are NOT left behind, or tied to the bumper of the car.
Johanna and I had different driving techniques. Her cat cage was more protected from the sun and she is the 'stop at a park, eat an apple' type of person. Therefore she took all kinds of time making her way to the next stop. [We had to do 350 miles per day by the terms of our job relocation contracts; this was about 7 hours on the road daily.]
I was in a hatchback and it's long sloping window generated a lot more heat; I didn't feel comfortable leaving the cats in the car, even with the windows down a bit, so I traveled like a trucker. All my stops were lightning fast to use restrooms, get gas, or fast food. Only once did I stop to take in an Antique Barn, but that was on a cool day in the north and I found parking in the shade.
Tip #5 - Plan your stop strategy to protect your cat's health. Make sure they have access to water. If possible, take turns staying with the cats with the A/C running while the other uses the restroom or gets food. In this day and age un-attended pets in a car might draw law enforcement and abuse charges.
I arrived at my motel room very early and did my relaxing after they were safe in the room. It was not the ideal situation but I managed all right. It was worth it to have them on board.
Tip #6 - Make sure the motel room is secured for your pets. If they are uncaged (and they will be to use the litter box and eat), be aware of their environment. Check the room upon entering for any holes or other hiding places or ways they can get out. Keep them leashed or caged when out, or at least, post a note on the door - "cats inside, do not enter" [better get that in spanish too).
I never had a problem in this area but Johanna found potential problems with more than one of her rooms. We don't want cats getting into walls. It draws media.
Scamp and Bubba became champion travelers, quiet and contented. I loved traveling with them despite the trucker methodology. We even stopped to visit with relatives in Pennsylvania. However, after all of this, I almost lost Bubba on Interstate 84 in Danbury, Connecticut.
All of a sudden he had ENOUGH with the travel thing and suddenly had to get OUT. There was a little space between the top of the cage and my part of the vehicle. He pushed out padding and got right up with me as I was driving at 60 mph on the interstate.
I pulled over and tried to get him back in the cage; that required opening the car door and pulling back the driver's seat. During this process he squirmed out of my arms and began running along the side of the interstate. Thank God, neither of us ran into traffic and he was so scared I was able to pounce on him and get him back into the cage. He was not very happy about the last few hours of the trip to MA, but we made it ok.
Tip #7 - Don't take it for granted if they settle down for the ride, they can get spooked or stir-crazy and try to escape. Make sure you are safe to do the driving and think through each time the cats are handled, transported, or let out. They could suddenly surprise you. Don't run into traffic or get distracted and have an accident.

Bubba, being nice.
Well, MrsGryn, I hope you are not changing your plans after reading this. I look forward to a successful trip. It will be memorable for all involved.
EDITED to answer comment(s):
There was a latched door built into the cage right behind the driver's seat; I could reach in, and they usually came to me. I opened the cage door, leashed the lured cat, and carried him to the motel room then got the 2nd cat; in the mornings I reversed the process to load them up. The opening was large enough for me to access them but small enough so I could block the way.
I could also access them from the hatch [the hatch!], but that was a lot more dangerous. I think I had to do that only once when Scamp refused to come near.
I traveled light in the way of my own luggage which resided on the floor of the passenger side and in a little in space behind the seats. The cats had a litter box below the hatch deck where you see Scamp, and had a food and water bowl too.
As for that turtle, averaging about 50 mph is pretty good particularly in the east when you hit traffic. Also, I was the only driver and so there were short rest stops in the car besides the potty-food-gas stops. Two drivers, and the 65 mph speed limits these days will probably speed things up for you.