Several times each driving day I took the time to get off the interstate and find a nice lake for a dip or field for a run. In Wyoming, Frankie was in heaven because every stop offered a pristine lake for him to swim in. Remember, the books on tape just don't cut if for the hound and he gets bored. He wants to run, fetch, swim and get attention. And the scenery goes by too fast for him to get much out of it.
Interstate rest stops always have a dog run, watch the traffic.
When stopping for food at those enormous truck stops, it can be hard to find shade to park in. But I found there was usually shrubbery planted around the restaurant. My human travel partner would go inside and demand a window table with a shrubbery view while I secured Frankie in the shade under the bushes with water and a biscuit. Never, never leave the dog in the car in the sun. Don't let hot concrete burn his pads, or sharp gravel cut them.