I usually open both the rear truck window and the connecting window from the camper. I have a foam "donut" to seal the gap. If the seal is not good I get whistling noises. The donut is essential. Without it the dead space is gone and I would not be surprised to have wind buffeting sounds and bugs/water/dirt enter the space and even go into the camper.
Anyway before attaching a deflector to the truck or the camper, I would try some experiments. Seal the gap between the cab and camper with foam or perhaps with a canvas dropcloth and see if that helps anything.