Welcome to the forum Laura.
Hot and dry sounds nice but may be your problem. The mildew you describe on the cucurbits is, I think, caused by dry-weather stress when roots have too little moisture to maintain healthy growth. Water less often but thoroughly and mulch with some well rotted organic matter if you have some, although ideally you will have done that in winter or spring preceding.
As for the tomatoes, hard to say without seeing soil condition but it could again be some kind of moisture stress from lack of organic matter or uneven/insufficient watering, in view of the weather you describe. Watering takes time to learn and having soil rich in organic matter, as a moisture-store, helps even out any irregularities when we water.