Not sure if you are talking about the mud flap itself, or the fender well liner.
The mud flap directly behind the tires is the best place to stop the majority of the rocks that can do damage to the undercarriage and do eventually find their way to your tow vehicle.
When you replace them, make sure you make them to hang within 6" of the ground when aired up. We usually install them to be about 4" off the ground when possible. You do not want them to be on the ground when you let the air out, so some coaches need to stay 6" up.
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