Only issue with grades and switchbacks is going to be the approach to the tunnel in Zion and the tunnel itself. Check the Zion NP website for restrictions. They will charge a fee for the permit restrictions on length or height as they close the tunnel down for one way traffic. Many many rv's go through there daily just depends on your specific rig if it fits
https://www.nps.gov/zion/planyourvisit/the-zion-mount-carmel-tunnel.htm
other than that 9 is fine a bit narrow within the park, 89 and 12 are fine roads and should present no real issues to Cannonville. In fact only other significant grade I can recall is 12 coming down off the plateau by Bryce to the valley bottom and again the road is fine.
In the summer time the tunnel they typically just close traffic one direction for a period of time last one through takes the baton to the other side gives it to the ranger and then traffic is allowed to go the other direction for x period of time or until they run out of waiting vehicles and again last person through takes the baton back to the other side. I have never had to wait more than 15-20 minutes at the height of summer traffic though it has been about 5 years since I was there last.