I'm not in the same place or climate. I do try to keep gear when in the vehicle out of the sun in the summer, and to limit the amount of gear I have with me on speculative trips where there might be photo taking temptations.
I don't ever have it hot or muggy enough to worry about fungus growth. I do when vacationing keep my film stash in a soft sided cooler, but without ice. I usually bury it under a pack of sleeping bags, etc, but not right on the floor of the trunk. That way I figure it misses the worst of the days hot , and gets a chance to cool off at night with most of the gear off the top of it.
Mostly I don't worry about it, hot or cold. If the gear gets hot, well, move it to the shade til it is cool enough to touch, then start using it. Take it for CLA's as soon as it starts to stop working right. Shoot a film, and get it processed, rather than having 3 cameras on the go with film in some for weeks on end.
If is gets too cold, pack the gear it in a zip lock or two, then warm it in a heated place, so the condensation will form on the zip lock, not the gear. Take the batteries out and stick them in your pocket until they are warm enough to work properly. Dont over warm gear that is to go back into the cold or it ill frost up again. Advance film slowly and avoid power winders to minimize static spark risk on the film, and another battery pack to keep warm.