Exercise.
More exercise. MORE.
Take them to dog parks en route, take them on long brisk walks at the campgrounds. When you stop for gas or groceries, take them on mini walks.
Exercise and more exercise!
It cures just about any stress in a dog. Keeps them busy and focused.
I travel with a dog. Once I incorporated MORE and more exercise he's been fine and he LOVES it. Get those dogs out on long walks. It gives them a chance to focus on all the GOOD stuff. Dogs love poking around sniffing up the new neighborhood. Could be just too much time in the RV and not enough exercise outside.
I search out dog parks and often stop there instead of rest areas when I am traveling. It's fun! I've had many picnics in the RV at dog parks too. I exercise the dog, then take him back to RV make our food, then if time, run him around the dog park again, then back on the road.
When I come outside from grocery shopping, I toss the stuff in the RV, then I leash up the dog to go on "cart return". I walk him briskly back to the store front, pushing the cart all the way. He LOVES this now! It gives his legs a good work out, then we go riding again. My friend traveling with me was laughing himself silly when I told him the dog had to go on "cart return". But it works and it makes the doggy super happy to do this.
I've also stopped in the large pet stores on hot muggy days and briskly walked him up and down every aisle in the store, sometimes several trips around the store. Then we buy a little food and off we go again.
All the "mini walks" gives him plenty of ops to do the bushes too. He often does not eat while we are traveling. It's his choice. The food and water are there on a rubber mat for when he feels like attacking it. Even at the new campground he is excited for his first walk around the park. Then often when we get back, he devours a mountain of food then happily collapses for a nap.
So often now, I just hook up the utilities, then it's doggy walking to see the new campground. It settles him down and no more tummy trouble.
The answer may be as simple as MORE MORE MORE EXERCISE.