You can tell the difference by the noise! It is very noisy when blowing air straight out of this vent and much quieter when blowing air through the other vents.
I installed an automatic Maxxair 0007000K MaxxFan in the bathroom and set it so it exhausts air after a certain temperature is reached in the trailer. I can leave the trailer all day and it does it's thing while I'm away making the temperature a little more palatable on hot days. I leave the 2 air vents partway open also except if I think rain is coming when I close them up or nearly so. At night the Maxxair is a fairly quiet fan when set on 20% and this plus having the bedroom window open a couple of inches with the handle jammed by a half clothes peg keeps the rig somewhat cooler and at least enables some airflow.
Trial and error will find what works for you.
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