In addition to what Lori posted, toppers are valuable to keep the heat from radiating into the slide. My coach is easier to cool because of the toppers. My kitchen slide has no topper. If I forget to open the main awning enough to cover the slide, the inside of the cupboards really heats up.
For me there is no MPH of wind before retracting the slides. It depends on how the wind is hitting the slides and the amount of noise they are making. If they are making enough noise to make sleeping or watching TV irritating, I will retract the only offending slide.
2005 Newmar KSDP 3910 + GMC ENVOY XUV 37K lbs Moving Down The Road
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