If I am pulling on a highway or good paved road and want to cool down the inside of the trailer, I might open a window or two at the last stop before the final destination.
Otherwise, I usually leave them closed.
I have a MaxAire cover over the vent on ours so I generally always leave the vent open during the whole camping season, unless it gets cold out at night while camping.
You can see the MaxAire in the photo below . . . large white dome in front of the air conditioner.