The RCW was amended in 2014 to change "in or upon a motor vehicle" to simply "in a motor vehicle."
You could argue that an exposed bed is not "in" a motor vehicle.
It's still not very wise.
I gotta think you would loose that argument.
If you were driving down the road and your loaded gun flew out of the bed through the window of a car behind. I am guessing you could be charged with insecure load.
Because the firearm or any object was IN your truck.
I a guessing any game agent or LE would site you for loaded in the truck.

Can you imagine how many accidents there would be with guys caring loaded long guns in the bed of their trucks while hunting. Allowing that would be nuts.