Hwy 395 Hwy 120 and 6.... are they good for a 42' 5th wheel

I have done that very route in my 35' DP without problem. It's a nice scenic route and parts of 6 and 95 are so remote you don't see other vehicles for an hour at a time. It's all good roads, but there are curves and hills.

I just checked the Caltrans website and the section of 120 between 395 and 6 is restricted to 40' max vehicle length. So it's up to you if you want to chance it or not.