I have not seen any RCWs that state that. I know, not much help there. However, I'm sure employers wouldn't mind you leave it in your vehicle. Most of the businesses that tell you it's prohibited in their store will often tell you to "leave it in the car" - so I don't see why the same principals wouldn't be applied for employees.
Besides, what about "don't ask, don't tell"?
I don't know of any employers who request to search personal vehicles.