I had a similar problem on one of my other coaches. I fastened a piece of 1/8" stainless steel cable to the bottom center of each flap, ran the cable up at an angle to the frame and fastened it to the frame with a slight amount of tension to pull the flap away from the tires slightly. No way for the flap to get in the tires now.
You could also look for heavier and stiffer flaps, your factory flaps might be to flimsy to stay down properly.
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