Thousands of RVs make that drive every year, so it isn't that dangerous. But it has what rock climbers call exposure, you feel like it is dangerous. Mainly because you can see too much for some people. Especially if you are not used to that sort of road. 7% grade is just about standard on mountain roads and Hwy 12 in Utah has a 15% grade section and thousands of RVs make that trip every year.
It's mostly in the head! It's funny how differently we perceive things. In all these years traveling the mountain West, I've never felt comfortable towing our fifth-wheel between Silverton and Ouray, though I have driven that route many times.
By contrast, I had no hesitation whatsoever about Utah 12, and have recommended it many times as one of the best drives in America--and my experiece was towing a 34' fifth-wheel. It was a delightful experience and I felt totally confident.
Now for the confession: my wife insists she will stay in Ouray if I set out to drive the Million Dollar Highway with our fifth-wheel. I think that's the real reason I haven't yet tried it.