Jim,

The month of May is an excellent time to visit southern Utah, and thereby
avoid the heat of Summer; sixties and seventies during the day, but be
prepared for cold nights.  If you go to Bryce Canyon N.P. you'll be at 9000
feet, at which altitude you may experience sub-freezing temperatures
overnight in May--nothing to be concerned about, just be prepared with the
proper gear.

Eight days may seem like a lot, especially if you've not had but a couple
of days off for a number of years, but don't underestimate the distances
traveling in the West; you didn't say how much traveling you've done in the
West, but many Easterners and Europeans plan trips out here without
understanding the size of the country.  They try to see too much in too
little time, and the vacation becomes more of a burden than a break.

I'm guessing that Hollister to Zion National Park is about six hundred
miles, and it will be easy to add another six hundred miles, and more,
rather quickly in Utah.  You could very easily spend half of your eight
days sitting in the van, and as enjoyable as that may be in the rig you'll
be driving, you won't really experience these special places in Utah unless
you get out and hike.  Get out early in the chill of the morning to enjoy
the low-angle light and shadow on the red rock, when you will also
experience some degree of solitude before most others are out and about.
Similarly, evening, sundown, and the twilight afterward can be magical.
These hikes will be what you remember, not the miles you drive to get to
the next "attraction".  Pick a couple of places, and spend the time to have
a good look around, and save the rest for another day.  Experience the
magic, and you WILL be back.

Bill