I was fortunate with my old dog. It was just her and me when I got her as a pup, so no distractions in training and only one voice for her to listen to. No one to spoil or ruin her. Had a water district job at the time... Management didn't care if she rode in the truck bed on a short chain screwed into a toolbox or in the cab with me on my agricultural water route. So she learned to ride, relax and behave early on in her training. She also learned to sit when there were people or livestock around unless I said otherwise, which really helped after moving into deer country and goin' on shed hunts with her (pictured).
From then on during any travel - visiting family or friends, camping or fishing in or out of state - she went everywhere with me without trouble. I carried a leash and a long synthetic tie rope with a ground screw if needed, but rarely did. She slept with me in the covered back of my pick-up, in tents, in hotels, in vacation rentals without hesitation or issue. She was the best. Even her vet said I'd never find another like her.
Some of it was the intelligence and trainability of her breed (border collie x heeler) and some was her individual personality. But a large part was being able to have her by my side daily since Day 1. The more you can, the better they turn out.
In comparison, my newest rescue (also an intelligent Aussie x heeler mix) was a burned fire victim who was never worked with in the first year of his life. So uncared for that his family left him to die when wildfire hit our area. He's more of an (ahem) free spirit and a challenge to travel with. But he's a good guy, gentle and eager to please, and is gettin' better every day.