Hey Carrisa, I would not suggest the Rockville route without 4WD and good ground clearance. The longer way around took about 35 minutes and is mainly flat dirt roads. Have fun!
It is south of Zion and yes on the way to Rockville. You will pass over a cool old bridge and eventually veer to the left on a dirt road. We have a revel van with 4x4 and some people who have big rigs without the boost can camp in the lower area in a few spots and just look wistfully up at the road which leads to greater heights and vistas with amazing views.