I am in the tropics of south Florida and this has always been an issue. I do not have time to look it up right now (maybe someone else will), but someone on this board or a related board did a scientific study using food packed in Igloo-type coolers in southwest Texas inside their car to determine if the temperature issues could be managed. In short, they could not without judicious application of electricity and expensive technology.

I do not believe it is so much the temperature fluctuations as it is the high heat that you should be concerned with. So during the winter months, this should not be a problem.

Of course, the other thing that you can do is rotate out the food that may have been exposed to heat much more often.

In short, any food I keep in the car gets carried in and out of where I cam going which substantially reduces the amount I keep around. If I am planning on going off the grid, that amount changes substantially.

Mainstay probably has the best resistance to heat of the non-dehydrated foods, but like you say, not a preferred option.