Hey, I was looking at this and sort of asking "What If?" regarding the pressures involved here. If I'm not mistaken, there is going to be a high forming along the roof of your trailer due to it being above the curve. And below the trailer there is going to be a fairly deep low. When these 2 meet finally in close proximity, I fear a wiggly vortex (Aerohead has a better term for this, I'm too lazy to find it again) will form and be noisy while robbing you of some efficiency here. Not sure what the solution is. I'm hoping Aerohead looks at this and has some insight...(yeah I went there) as to: A) If my fear is justified. B) Is it significant. C) What can be done to mitigate.
Higher Res Version
Here
My only thought on mitigation would be to build down below the rear of the trailer behind the axle. This would serve to have a greater separation between the 2 pressure zones and reduce the strength of the low.
I hope this makes sense, my area of concern is that place 12-18 inches behind the back of the trailer where the red & blue are about to meet. I have always gotten a little worried when 2 areas of differing pressures are close to each other.
I sort of used a more distant shot of an Insight for analysis in my illustration here. It is to scale top to bottom and the trailer gap is the same.
Also I might suggest that you build as far forward as possible on the front of the trailer to reduce the gap some more.
Your project looks great, I hope this helps you a bit. Where you at in FL? I'm guessing panhandle by the trees (no palms) in the background.