And, there's some useful advice on the thread Missouri to Moab.
It'll be steaming hot at Arches, Canyonlands and most of southern Utah. But there won't be any steam, it's too dry. It's 20% humidity at my house today and I'm 100 miles east of Utah where it's drier. You won't know you are sweating because it'll evaporate. You'll lose a lot of salt and can get weak and woozy. I used to take salt pills at times, but modern sports drinks have better balanced minerals and electrolytes.
Boondocking is difficult because no A/C. If you are up high enough, it can get into the 50's at night, even lower. It's supposed to be in the upper 80's here next week (but 32˚ tomorrow morning—typical crazy Colorado weather) at 6,837'. Utah is lower, but there are some higher areas—beautiful drive through Boulder, Utah and surrounded by national forest with campgrounds.
If it's too hot for you in Utah, the North Rim of the Grand Canyon is cooler than the South Rim by several degrees and Colorado high country is usually cooler than elsewhere around here. Frankly, I'd never go to southern Utah in June-September. It's an oven.