I built an air deflector for the back of my flat back TT that takes the air coming off of the roof and directs it down the flat back to break up the vacuum which helps it release from the air flow. Works good. I got the design off of the internet after googling "flat back vehicles" or something like that. The back of the TT stays cleaner too since the dirt doesn't curl back and stick onto the back siding. Years ago I used an air deflector on the back of the TV, don't think it did as much as my rear deflector on the TT. The TT also tows much more stable without the turbulence going on behind it.
Notice that airplanes are fat in front and taper to a point in back.