Not sure you would want to introduce more road grime and dust into your camper with the side windows open.
If your window is a emergency pop out window, I wouldn't trust the slider bar to stay in place.
If your window is a tilt-out - I can see it inherit some unintentional aerodynamics and pop out of place.
My suspicion is this rumour was started by a rv window manufacturer trying to drum up new business.
I've been instructed by the rv dealer - always - always keep windows closed while towing. I didn't ask why - sorry.
The dealer did sell me two Max Air covers - for the rooftop vents - and encouraged me to keep the roof vents open while towing - using the Max Air covers of course.
ymmv