We just got back from 2 weeks on the coast in 90+ degree heat. Our dog was with us. I was curious, so I kept a thermometer at her crate and checked it every so often. If we woke up to a 70 degree trailer, shut the air off and hit the road, it was 5-6 hours before it hit 80 in the trailer (this is with outside temps around 90 and in the sun on the interstate).
This tells me that in most instances, if the power went out, our dog would still have 5-6 hours (in our trailer) before it got "warm". Like mentioned above, she lays in the driveway in 90-100 degree heat. Therefore, we are comfortable leaving her in the camper with air running and we will leave for as long as 5-6 hours. If conditions are shady and the temps below 90, we would leave for as long as we wanted.
I prefer to leave her at home with friends...