Our dog had bad car travel stress when she was younger (she's now 2.5 yrs). We pretty much let her choose where to sit - either in a front seat passenger's lap or she'll check out the back seat floor. The only calming thing we found worked is Bach Pet Rescue Remedy. It takes a while to work, but it does work.
https://www.amazon.com/Bach-Original.../dp/B000RFTCZW As she's gotten older she doesn't need it. In the motorhome she rides up front with us teethered so she can't wander and get out of sight and has a piddle pad, food, and water available. She settles right down.
For air travel our vet gave us Aceproxamine. We give it 30 minutes before we take off and sometimes give a boost with the Pet Rescue Remedy. She's small and has to be in a carrier for the entire flight and under the seat for take off and landing. It works pretty well but not perfectly and if we have a flight delay it's hard. We learned we have to give both with food and water or it doesn't get into her system. On airplanes, a lot of our success depends on the flight attendants. A flight attendant who doesn't like dogs is a big problem. The dog senses that stress. Also, a nice attendant will let us hold the carrier in our laps so we can sooth the dog. It's hard, but kenneling is worse.
The RX takes time to clear out of her system. So, she can be a little sluggish the next day. We make sure she gets plenty of water but I think it clears the liver slowly. It's not our favorite. You might try smaller doses for an older dog.
With our previous dog (a mini dachshund) travel was harder although as she got old she just became more lethargic. I guess we were lucky in that. Hope you find the right combination.