Seattle Area I-5 South to Carnation

Best would be turn east from I-5 on US 2 in Everett. Then at Monroe turn south on SR 203 to Carnation. Worst part would be a couple high traffic miles entering Monroe. SR 203 is a good two lane highway. Not much for shoulders.

You guys nailed me when I lumped the Seattle area from further north. I'm starting from the Swinomish Casino and want to use the "best" route for a large MH with toad.

Agree about Google. It's good for multiple routes and traffic in some areas. Not always accurate and no RV related road information that I'm aware of.

I live and worked in the Everett-Seattle are for over 30 years. I would not trust Google or in that matter most GPS systems. Going east and west out of Seattle to get to Carnation can be task.
Knowing where you are coming from will definitely help especially with your rig.

Carnation is almost due east from Seattle not south, Evert to Seattle 30 miles or so. I lived in the area 35-40 years, just where are you starting from?

For a route that short, you can use Google Maps and specify "Depart At" and put the date and time you want to leave. Google will estimate the trip and give you options.

Really depends on what part of I-5.
One way from central Seattle area, would be i-90 to Preston-Fall City Rd, then north from Fall City to Carnation on SR 203.
From far north, go to Monroe on US 2 or SR 522 to Monroe then south on SR 203.
Avoid Seattle!