I don't know which roads you're driving to get to the area but if you don't want to drive Hwy 9 coming from the east via the tunnels, a very easy, scenic way is to head a little further south on Hwy 89 and go to Fredonia, Arizona and pick up 389 west which becomes 59 in Utah. Take it to Hurricane, UT then east into Zion. No hills or tunnels to worry about.
If you're on 89 another way is to take Hwy 20 just north of Panguitch, UT over to I-15 then south to Hwy 9 just north of St. George, UT and head east to the park.
From Hwy 89 in Utah just north of Hwy 9, don't take Hwy 14 to I-15. You won't like it! Steep!!