My fender skirt(s) have also cracked in the narrow area over a tire.
I got my replacement from Forest River because my dealer is many
miles away. I figured the original had cracked because they used pointed
sheet metal screws and had not pre-drilled holes in the plastic skirt.
Mine had cracked right thru a screw hole.
I carefully drilled holes in the replacement skirt and caulked it in place.
It cracked within a year- just not directly thru a screw hole
I took a piece of copper sheet and made a patch which I placed in behind
the crack. I fastened it with aluminum pop rivets and it's not at all obvious.
I guess the plastic is cracking due to thermal expansion and contraction.
Mine cracked on the sunny side first but the shady side cracked a couple
years later. Of course you could use aluminum or stainless or
galvanized sheet metal for the patch. I just happened to have some
heavy copper flashing so that's what I used.