As a kid we rode hundreds of miles in the summertime in the back of a pickup with a topper, and it was never too hot, we'd have the side vents open for some breeze, and there was always the little window to the cab of the truck - sliding windows in both the topper and the truck window - that we generally referred to as the "woofle hole" as it was the right size for our Saint Bernard to stick her head through, and then later, our Great Dane mix. Rode back there in the winter, too, and it was never too too cold, particularly with a large furry critter with us! We never did thousands of miles back there, but i imagine it'd be pretty much the same!