My last couple of pick-up trucks did not have a slider window in them. But we had one with a slider several years ago, and the shell we had on the truck also had a slider window. Back then, we often carried passengers in the bed of the truck, since the truck was just a front seat truck. We wanted to leave the slider windows open, but like you, experienced the wind.
I came up with the idea of using a bicycle inner tube. I positioned the inner tube around the window and then, with a hand pump, pumped up the inner tube and it sealed around the windows, making the window a pass-through.
Occasionally, I'd have to preposition the inner tube, and sometimes blow it up again. Eventually, I swapped out the bicycle inner tube for a motorcycle inner tube and then I never had to mess with it again.
Anyway, it worked great for us. It was cheap, and only took a bicycle pump to install it. It worked on that truck and shell for about 10 years, and then ended up getting a different vehicle.
It would probably work just as well for a truck and truck-camper. However, unlike a shell, there might be a greater distance with a truck camper between the two windows, and a bicycle tire might not be fat enough to make contact with both.
Good luck!