In the RV park with our ACE, we would hear knocking occasionally when receiving strong winds toward the RV rear, I thought it could have been from the refrigerator vent (like a swinging vent, or awning, - It was kind of in the area of the shower). Since high winds are accompanied by rain and cold temperatures at night and I work during the day, I never isolated the problem until today.
The rear tires have a mud flap that is reinforced by a bolt-on T-shaped bar. Strong gusts would push the flap forward and the reinforcement would allow it to swing back against the under carriage causing the knock. I slightly bent the T-shaped bar toward the front of the RV increasing separation from the chassis. Noise gone!