I agree that much has to do with the individual dog's personality, and the level and type of socialization the dog receives. You certainly can overcome negative experiences, but most people don't want to have to do that, and especially if they haven't had significant dog experience. And I think it's good to stack the deck in your own favor. All that said --
Our Shih Tzu is an excellent traveler. They're low energy and are commonly said to be the least barky of the toy breeds (which is absolutely true for him--he's a very quiet dog). He gets in the RV, curls up and goes to sleep. He's always eager to explore wherever we stop, but when we hit the road again he goes back to sleep. He almost never barks. He's very friendly to people and other dogs, but mostly he only really cares about me and so doesn't see the need to be bothered by other stuff. He only needs short walks for exercise, so it's not hard to tire him out for travel or when we'll be out sightseeing. Although he doesn't *need* a lot of exercise, in good (cool) weather he can easily handle three to four mile walks or hikes. You do have to be very careful with them in hot weather, though. So a good genny and AC is a must when traveling in the summer in most areas.
And this is perhaps an exception, but our former beagle was a very good traveler. She'd also curl up and go to sleep as soon as we hit the road, and rarely barked. She was pretty darn lazy.
My concern with another Sheltie would be that they are known for being one of the more vocal breeds, which of course can be challenging to deal with in a campground. And many I've met have been rather high strung. Although I will say that one of my best friends had one that I believe would have made an excellent RV dog. She was calm and easy going.
Good luck, and thank you for doing your homework!
