We had a bad crack in our shower floor in our '87 Award TT long ago, Plastic pan, likely polycarbonate. Filled the void under the shower with high density spray foam, the kind intended to fill small gaps around windows and doors in houses to give it support, then ground and sanded the area to a feather edge, repaired the damage with an auto body 2 part putty laid in with fiber glass window screen. I dont remember the name of the product.
Go to an auto body supply house to get the product.
years later we sold the Award privately for top $