We have the old style glass louvred windows. They certainly aren't very air tight.
We often run with these slightly open. Usually the ones above the cab.
Our rear door is really badly sealed also, so there is no concern about pressure build-up - attempting any sort of air leak test by pressurising our camper would be futile unless pressurising it using a jet engine!
However, driving along the slightly open cabover windows must cause a slight pressure increase and a flow through the camper. I've mentioned this before, but driving through miles of that fine dust you get in parts of the Sahara we were amazed to find no dust penetration at all. In Namibia we had a vehicle that sealed almost air tight, and dust still got on every surface, but in the Sahara - nothing. Must be the positive ventilation effect (only clean dust-free air enters the over-cab windows, and the pressure of that prevents the dusty air behind the camper being sucked in through the leaky rear door and windows).
Note though - If I had those modern plastic Windows I would be very concerned about leaving them even slightly open for fear of being torn out.
Steve.