All of the stated roads are very doable with a 40' MH and toad. Your motorhome would be very capable of doing it. We've been on them multiple times. However, we enjoy mountain driving. If you haven't had any experience with these kinds of roads you might not like it. However, by the time you get there your confidence level might have risen!
๐ Hwy 12 is the one that seems to bother folks but it's a good 2-lane road with pull-offs and small towns along the way to take a breather. Everyone drives slow so there would be no big lines behind you and thousands of RVers have driven it. It's really a very pleasant, highly scenic drive but no one can really tell you if YOU can do it. It's how YOU feel about it.
You might consider purchasing online the Mountain Directory - West. It's about $18 and well worth it. We use it all the time. It will give very detailed information on these secondary roads.
Have a good trip!
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel