Even if they did work which is very doubtful for most truck/5th combinations, they hurt your mileage when not towing which is probably the majority of the time.
I knew several guys who had jobs towing 5th wheel from the factories to the dealers and from the dealer to RV shows. Most of them had the deflectors at first but over time most removed them. When I asked they said that they gained little in gas mileage and while they did raise the bug line higher that was not necessarily an advantage as they were required to wash the 5th before delivery and a thick line at the top took longer to clean.