As Athearn notes, most National Register photographs are taken with 35-mm equipment. That includes me. I never leave film of any sort in a parked car or in motorcycle saddlebags in the sun. I would suggest you bring along a small cooler with a little ice or another coolant inside it in which to keep your film on hot days. If you use ice, remember to wrap your film in a watertight package to protect it.
Humidity can also be a problem for film stored in an ice chest.
Always try to keep your film as cool and dry as possible when outdoors in the summer. Such lenses correct for the visual distortion found in architectural photographs taken with normal, nonshifting lenses. Without the use of a PC lens, buildings in photographs typically appear to be tilted backwards, as though falling over. Currently, Nikon, Olympus, and Canon systems have the flexibility of offering multiple PC lenses in more than a single focal length the standard 35mm plus a wider lens. Several other manufacturers produce PC lenses for their SLRs, but just in one focal length.