I have driven that stretch once, about 3 years ago. I don't know the exact drive time, Google Maps says 2 hrs 48 min to cover 117 miles. I would assume that is in a passenger car and it acknowledges the fact that even in a car you can't average 55 mph. I would probably add another 30 minutes to that in an RV.
Are there switchbacks or any other things you should be aware of? Why yes there are. That section of road contains the "Hogback". See here: http://www.scenicusa.net/021110.html. It's a very scenic section of highway, which drops very sharply off on either side. It's either very beautiful or terrifying for the passengers, depending on their level of trust in the driver. If you're the driver, as I was, you can't take your eyes off the road to see the beauty so its mostly just scary. If I recall, there are quite a few curves and maybe a few switchbacks as you start descending. I did it in a 37' DP towing a Honda behind. It was definitely an interesting experience.
Regarding 89, I have not driven that way, but based on my view of the map, I don't think you would go all the way back to the interstate. Google maps (actually Bing) says the 89 route is 21 miles longer than 12, but calculates drive time at one minute less. Maybe others can chime in on that route.