We traveled ten weeks last summer with our cat, and we've done several weekend jaunts since. She was great. We have a little harness for her that we used occassionally depending on the situation. She loved the Airstream; in fact, we joke with a bit of seriousness that we bought the Airstream for her!
We put her litter box in the back seat of the pickup and let her have free roam of the vehicle. Most of the time, she sat in my wife's lap or rode on the armrest between us. Sometimes, she would stare out the back window and howl like a wolf at the Airstream.
She developed a habitual need to vomit every time we started towing, so we learned to not feed her much before starting driving, and then we'd stop soon after to feed and water her. Her need to vomit was so predictable, that one morning we had literally only been under tow for 10 feet when she let it go. We learned to just have some paper towels and plastic bags ready, and then it was smooth sailing from there.
On a very sad note for us, our wonderful, perfect cat passed away this morning. We will miss her sooooo much! So our advice is to go Airstreaming with your pets, treat them like family, and learn how they like to travel. In the end, they probably just want to be with you.
She was such a blessing for us. The last two pictures were taken just this past Friday, March 21. We took our Airstream to the beach for the day and had a BBQ in the evening with friends, one of whom brought her dog, Lilly.