Searching for "Best washington diesel mechanic" etc, here are a few shops or peeps claiming to be satisfied customers. I make no claims that they're any good, haven't heard of them.

http://www.thepowershop.com/index.php?pr=Ford in Enumclaw.
One of their mechanics has a good intro to his work:
http://thepowershop.com/index.php?pr=Combatking

And some RV'ers like him: http://www.rversonline.org/ArtPowerUpgrades.html
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http://tinyurl.com/36qxv6 Duvall looks about 35mi north of Enumclaw