You could have been more adventuresome and taken Hwy 20 in Washington State heading east, now that road is really crazy, would not want to take that in an RV though (not even sure they allow RV's on it).
Here is a shot of HWY 20 after snow, I would not want to get caught on that road during a snow storm!
HWY 20 after a Snow StormWorst "main" roads I've ever driven in the US has to be that HWY 20 for the NW (if I recall), for NM it was probably between Cloudcroft and Alamogordo, and for Colorado it was the road to Telluride because there was construction and it was too narrow. For Arizona, perhaps just southwest of Prescott, the road through the mountains there.
Of course none of those are as bad as those 14k'r roads like the drive up Mt. Evans right outside of Denver, Colorado (but this isn't a main road)...
The roads in Wyoming and Montana are kiddie rides compared to all these, but add a little snow to any of these routes and they quickly become death traps.