Hey me too but I'll be starting in Moab and working my way down to Zion.
Yea, it will cool down to about 90 ;). Given the high elevation it does cool down considerably after sunset and is quite comfortable IMHO.
Last time there (yrs ago) I camped off grid and put more miles on dirt than pavement. No issues with heat or the few people I ran across, had a blast. Be aware of sudden thunderstorms and flash floods as August is monsoon season. Don't camp in a wash. I suffered from hyperthermia due to miscalculation of water on one slot canyon hike. Carry lots of water.