Traveling With A Cat

I have seen plenty of youtube videos of van Lifers, full time rv’ers, tiny travel trailer people, hikers, boaters etc. with cats. They are almost always on a leash.
Do any of you have a cat that travels with you? How did you train them on a harness and leash? Any tips or tricks? What harness have you been using that fits well and doesnt fall off? Or they cant wiggle out of?
I ask this because we had to go out of town a couple weeks ago last minute. Even though I had my son check in on him , it stressed him out a bit. When we got back he was puking and it had a tinge of pink. The vet tech said that he was probably stressed. After we were home for 24 hours, he was fine.
So, I would like to train him to go with us on our short outtings. Any extreme outtings and one of my kids will have to stay the whole time. But they arent always available.
I started him on the harness today. Normally he would flop, but not today. He would also be traveling with our dog as well. They are buddies .That might make the traveling a bit easier for him. I also have a kennel that they can both fit in as well. The cat weighs 15 lbs and the dog weighs 14 lbs. Best Buds!! We will have them in the truck with us when driving from one place to another.
Any tips tricks and advice would be great!