[aerohead]

Dr. Wolf-Heinrich Hucho ran Volkswagen's climatic wind tunnel for 10-years. He personally tested everything known in his day, and reported that the only two technologies that ever demonstrated meaningful drag reduction were either boat-tails, or box-cavities.
I know of no vehicle which ever benefitted from VGs unless there was 'more' body downstream of the VGs, at which point the flow could reattach to.
The original 'Air Tab' was for a Honda Accord notchback, and helped the separated flow reattach onto the boot (trunklid).
The Mitsubishi Lancer EVO had factory-engineered VGs to provide for reattachment onto the boot of a notchback car.
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There's nothing to reattach to behind your truck and any claim that VGs can help you are extremely dubious.
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There are some boneheads that have been driving through New Mexico with bi-wing foils extending beyond the rear sides of their 18-wheeler's trailer van with the expectation of a benefit. Hucho tested a monofoil version and this was also a fail.
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I would recommend you track down, online, NASA's 'shoebox' and 'bat-truck' project,from the 1970s, conducted at Edwards Air Force Base,California.