I camp with my Twoonie. Here is my advice:
First, it takes a bit of time for a cat to get accustomed to the motion of the vehicle - usually about 20-30 minutes. So if you can do practice, make a couple drives of at least an hour to see how the cats adjust.
It is very much an individual thing as to whether each cat will prefer to be loose or in a carrier. Some cats prefer to hide away from everything. They don't want to see out windows - that movement just upsets them. If oneor both of yourcats is like this, you can get a "cat cave" type bed and tuck it in a quiet corner away from the engine vibration. Other cats want to see out the windows. In that case, you can set up a bed in a more open area.
ALWAYS have good fitting harnesses on the cats at all times. Before opening any door, attach a leash to the harness. Cats can slip past you in a heartbeat if they are startled.
Get your cats used to their harnesses now before the trip. Let them wear their harnesses for an hour at first, then increase it up to 24 hours as they adjust to it. Some cats take to it easily, others fight it. Don't attach a leash until they can hang out inside wearing the harness.
I use the Yellow Dog Design brand roman harness for the Twooney. With Looney1, it works great. He is a stockier build and has never slipped out of it. Looney2 can easily back out of it - he is sleeker and pulls his shoulders into his body to slip out. Because of this, I actually put a second harness on Looney2 as a security measure.
Get ID tags for each cat. Include your cell phone number, RV license plate number AND car/truck license plate number. This way if you stop at a park/campground with spotty or no cell service, someone finding your cat can locate you easily. Because Looney2 can back out of his harness when on a leash, I actually attach the ID tag to his second harness which is put on first. That way if he backs out and gets loose, he still has a harness with the ID tag on him.
NEVER EVER let your cats go outside without being on a leash or in a carrier. I hear people doing this all the time when camping saying "he always comes back". What they don't add is "until the day he got attacked by a coyote, bobcat, cougar, eagle, loose dog, raccoon, etc" or "until he got totally freaked out and took off", etc. Don't take chances.
I use the Petsafe crystal litter. For the two cats, I can use one bag per week - just scoop out the poop daily. It eliminates the smell, vaccuums up easily, and absorbs the pee. So much easier than using traditional or clumping litter.
I feed the Twooney a can of Fancy Feast in the morning and one at night in addition to dry food always available. This gets them used to a set mealtime and to being called. If one gets loose, this can help get them back.
Looney2 is a bit more nervous in the car, etc. My vet suggested Zylkene. It's a milk derivative. It helps Looney2 settle down and sleep during the trip. It doesn't make him drowsy, just helps settle him. I've tried the felliway, etc, and it has never done anything. Talk to your vet first to make sure anything you use is appropriate for YOUR cats.
It looks like you have a motorhome. I suggest you bring the cats out to spend time in the MH before the trip. Just let them hang out in it for the day. Watch where they hang out and sleep. Watch for areas where they can squeeze into that could be dangerous for them. If you can do an overnight with them, do so - it will help them settle in when you go on the road. If they are comfortable with the space, then adding the engine noise and vibration and movement won't be as stressfull.
Make sure you have carriers with you. Nothing is safer than a carrier and having them available at any time is priceless. I also carry a foldable wire crate when I go - it just offers an extra safety net if needed.
Good luck. Just remember that each cat is different. Don't expect them to take to travel in the same way. Give safe places for them to hide away. Always know where they are before you even consider opening a door. Make sure any compartments that open to the outside are secure and they cannot access them from the inside. And give them lots of love and cuddles as you go.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)