I agree with Jim that the dovetail will shrink, but I don't believe it will withdraw. It won't have a chance. The low pitch brings the heel of the rafter inboard of the plate on top of the truss chord. So all of the weight of the roof will be carried on the truss chord, at the center from the kingpost and at the heel of the rafter, in both cases creating bending in the chord. However in a properly designed truss the kingpost would be a zero load member, making it redundant. In a true truss the heel of the rafter is connected to the truss chord (well-heeled?:) with a very strong joint capable of resisting the high horizontal outward thrust of the low pitched rafter (hint: steeper pitch, easier job) unfortunately, all that weight on the chord a short distance from its end support creates a very serious shear condition and the chord will shear horizontally at the shoulder on the underside of the chord and the beginning of the dovetail tenon. (Note a second benefit of the steeper pitch is that the point load is brought closer to the post, perhaps even over the post....esp. if gunstocked....

Just because I'm paranoid, doesn't mean it won't fall down, so I prefer to stay away from tension joinery. If I were to build this frame I would turn the truss chords at 1,3, and 5 into beams shouldered onto the posts, sized for the kingpost point load at mid-span, the structural kingposts would support 2 ridges or one continuous ridge, and then you could employ principal rafter - common purlins, or a simple common rafter sytem, being carful in the latter to make the plate sufficient size to carry the rafters at 2 and 4 and plates properly joined to the posts.

Even better, make the kingposts ridge posts to the ground, way stiffer...even if you only put them on the gables the triangulation of rafter, ridge post, and ties would stiffen this frame nicely perpendicular to the ridge.

Horizontal shear on the underside of joists and beams supported in pockets, in my opinion is far too frequently overlooked.