The NP's in southern Utah can be quite hot in July. Since you are retired I assume your schedule is flexible. Why not visit them in the cooler months of Spring and Fall? The advantage of retirement is to go places when others can't due to school and work schedules. Spring in so. UT means desert flowers. Fall means aspens changing at the higher elevations.
Have you considered heading north in OR and maybe WA during the summer months? Gorgeous scenery and cooler along the ocean. Crater Lake, Bend, the Columbia River Gorge -- all places to visit inland. Then return via the coastal route, staying in some of OR's great SP's.
Wherever you go, one piece of advice. Don't rush. We often see those who we joke are on the five national parks in five days vacation. They're taking videos as they drive by the sights. That way when they get home, they can see what they should have seen when they were there. Experiencing anyplace is so much enhanced by seeing at different times of day and taking the time to stop and hear the sounds of nature.