Interior temperature sensor and alarm

We constantly travel with our dog (heck, the dog is one reason we buy vans) and we frequently have to leave her in the van while we do other things, like sight seeing or hiking where dogs aren't allowed on trails. And of course, we get worried about heat. We have an AC unit when we're in a campground, but units fail, fuses blow. She always has water and we can leave the windows open slightly. (Only operable windows are in the front.) We do not have a vent fan.

I'd love to put some sort of temperature control with a warning light outside the van, with a little sign that reads "Dog inside. If light is flashing call us at 123-456-0987. If we don't answer it's OK to break a window."

Anyone have any ideas? My cousin makes industrial thermometers and he's never heard of anything like it. I already have a smoke and carbon monoxide detector in the van, where can I get a temperature detector?