I imagine that if you make sure the hitch paneling starts at a set distance from the hook, and the paneling on the tow vehicle stays precisely within that distance, they can be very close together without ever touching.
The divide would be at the surface of an imaginary sphere of which the hitch point is the center.
A gap that small would not disturb the airflow much, and you would not need flexible gap fillers at all.
It does not have to cover the entire surface of the sphere; the plates just have to meet when going in a straight line, as when you're moving at speed it will always be in a straight line or very close to that.
I edited the picture to show what I mean.
The hitch point in read, hitch beam in blue, panel gap in black and imaginary sphere and radius in green.
The picture is quite crude, and cannot effectively show the sphere; but I hope it conveys my idea.
You'd choose the radius as small as possible, so that it just meets the furthest extremeties on the back of the tow vehicle to keep the extra paneling on that to a minimum.
Another view from above:
Tow vehicle and trailer are the black rectangles, the red lines are the additional side paneling.
As you can see this design allows the paneling to go straight from the tow vehicles side towards the trailer.
There needs to be a gap in the roof paneling too: following the surface of the sphere.
The only thing that disturbs it is the hitch beam, which has to enter the forbidden sphere. So the side paneling on the tow vehicle cannot cgo too low, or it would compromise tight cornering.