as a solo traveler, I do leave my dog unattended in my camper in a crate with proper climate control when I need to go somewhere where the dog can't go. Usually with a radio on to drown out some of the outside noise so that he doesn't hear every car door slam shut, etc. but I also know that he's not incandescently barking nonstop while I'm not there.
I would never leave him unattended outside in a crate or otherwise. There's way too many variables....sun could shift and put the dog in the sun to overheat. He would bark at everything that he could see walking past. If a bear or wild animal or rattle snake wandered through the camp site, that could end very badly. Someone could steal him, etc.