Sick Melonssquash

There are several possible explanations for your ailing crops. They may just be getting fried. Hot sun concentrated by black tires could increase water loss past what the roots are able to absorb, even if the soil is wet. That’s a good idea in cool climates, but Boise summers are warm.
Or, there are a number of bacterial and fungal wilts that are carried over from year to year in the soil. We can’t tell which one it might be without a close-up of the affected plant—and sometimes a cross-section of the stem, and perhaps a microscope, is necessary.
Finally, is the soil deeper than just the height of the tires? Most crops need at least 8 to 10 inches, and 2 feet is much better. If you’ve cut out the mulch under the tires and loosened the soil underneath, you should be ok with that one.