Just in case others come to this thread looking for Zion/Bryce suggestions:
For Zion, the Zion River Resort is a very well run private CG. It's a short drive from there to Springdale, and then you can take the trams all over the park. Also you can start in Virgin take a toad or your TV up towards Kolob reservoir and see parts of Zion that hardly anyone visits. You also have the ghost town of Grafton where the bicycle scene from Butch Cassidy and The Sundance Kid was filmed (good for a short side visit).
Yes, Zion is hot in the summer.
Bryce always captures my imagination and we're going again this September. Roughly 8,000ft elevation so cooler than Zion. We haven't stayed there camping yet, but Ruby's has always treated us well on motorcycle or by car. The hikes at Bryce are worth the effort - last time we were there was January 2013 and on a 6 hour hike we only ran across 1 other couple. Summers are naturally more busy.
I applaud the OP for scaling back the mileage goals for this vacation. Very smart move!