Unfortunately for you, there's no easy LEGAL simple route for you to take. You are legal on US 101 but you won't be legal on CA 128....Hwy 1 is completely out.
The only LEGAL way you can do it would be to take US 101 South to Petaluma, Ca 116 East to Ca 121 East and Ca 29 North to Calistoga. This route will add about 70 miles to your trip, (248 vs 180) but you'll be safe and legal. It's up to you if it's worth the risk of taking a shorter route, but you asked for the best legal route.
Here's the link to California Dept of Transportation Maps. Most of the connecting routes such as Ca 128 are restricted to motor homes less than 40'. Your tag will be a dead giveaway to any LEO patrolling any of those roads, and yes, they will cite.....sorry....Dennis
CalTrans DOT Motorhome Map
We can do more than we think we can, but most do less than we think we doDennis and Debi Fourteen Years Full Timing
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