I have mirrors like that on my motorhome, Had to replace the passenger side one last year after a tree jumped out of the forest and tore it off, tearing out the anchors in the process. There was no way that the new mirror screws were going to hold the mirror on. The I measured the thickness of the wall and drilled a 1/2 inch hole where each of the old screws were, making sure that I did not penetrate the inner skin. I cut a 1/2 in hardwood dowel into four pieces so that they were the same length as the depth of the holes( not all were exactly the same length) I mixed up some marine epoxy and thickened it so there was no sag , put some in the holes and some on the dowels and then pushed the dowels into the holes. After the epoxy had set up, I was able to install the mirror using the new dowels.