Could you fabricate a spout that would easily attach to the trailer?  If the tailgate could be chained so it only opens a little and unload into the spout that would then funnel it into the auger. 

I don't know if the trailer is tall enough when being unloaded to clear the auger, and it might require a gate of some kind to control the flow.  We have a smaller version on the red and blue trucks (see atached photo) but it if course is not as wide as the trailer would be.

I've been looking for a photo of my black truck and can't find one.  It has transport doors.  A previous owner built a gate in the floor.  It dumps into a spout that comes out under the middle of the doors.  The slide has a T-handle attached to it as well as a crank similar to what you see on a gravity wagon.  When I first saw it I was skeptical.  After using it I wish my other truck could fit one.  It's no good for cleaning out the corners, but makes it easy to take pressure off the doors to open them.  And the spout is not as wide as our other trucks so it is easier to hit the auger.

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