Living in the very hot desert of the southwest, closing up everything once a place begins to cool down is standard. Somewhat like a car a/c when the car is 160 degrees inside...when the car interior is very hot, initially it helps to have a window open (since the outside air is a bit cooler) allowing the hottest air to escape, then close the windows as the cool air cools down the interior to a temp cooler than the outside air. If there's any breeze at all with a rv vent open, my experience has been that it will overcome the power of hot air rising out the vent and allow hot air in.