Yes, N Harris has it right. Aerothrust with propellers doesn't work, not even with paper calcs.
Edison : Plan B : a FISH TAIL. Aeroshells are now standard equipment on truck cabs, but where is the companion TAIL that allows laminar rearward flow instead of the turbulence from explosive decompression around the rear edges?
A SOLID shell, like a clam shell with vertical plane-joint and hinges to the side...I once saw on a long, towed trailer in Utah. But a SOLID shell like that wouldn't work on your standard semi. Safety issues, still sticking out at slow speeds/stops, wider radius on backing turns, narrow loading ramps and a 4.5 solid shell swung to the sides....wouldn't work as a practical solution.
So, plan C : either a balloon tail like a curved pyramid or an open weave fabric parachute on a 9'x10' frame, 3"x3" steel tube, with edge umbrella ribs. The inner surface of the balloon presses against the back doors and the outer surface pooches out into the curved pyramid form. It has to be a THICK fabric though, thin balloons POP. At slow/stop it sucks back against the rear doors.
On loading/unloading the frame is swung 270 deg on side hinges to the side of the truck, or swung 90 deg and shoved forward on top and bottom rails on the truck side.
With the parachute the fabric is stretched rubber like a nylon stocking and upon stopping pulls back against the rear doors like a sagging stocking. It should have umbrella ribs at the edges. At 70 mph the natural low pressure pulls it into the curved pyramid shape.
Now, why do this? The SUCTION on the rear doors(dust layer = BIG CLUE)costs a BOX blowing a HOLE through the fluid maybe 1 to 3 mpg in extra fuel costs, from that turbulence from explosive decompression. This TAIL would eliminate most of that loss with LAMINAR flow.
Cost? About the same as the front aeroshell. I just sent this idea off to 8 semitruck manufacturers yesterday, with the sentence : FISH whisper : swish, swish...tapered tail, tapered tail...swish, swish...tapered tail, TAPERED TAIL.....