The parts of CA20 that I drove between I80 and 101 should be easy in an RV. The area around Nevada City and Grass Valley is busy urban 4 lane (or more), maybe even divided. Around the Sacramento River it cross rice country. There are modest grades climbing to the Clear Lake area, where it winds between lake and vacation homes. Some have suggested a faster route around the south side of the lake, but route finding might an issue.
The restrictions west of 101 are imposed by the state DOT on state highways. They don't have jurisdiction over city streets. The reason for the restrictions is that some curves (and/or grades) are too tight for long vehicles to drive without crossing the center line.
More details on WIki
https://en.wikipedia.org/wiki/California_State_Route_20