Jack,
I don't know if I replaced 13 inch mirrors with 17 inch ones, but I did replace the factory mirrors with larger Ramco Mirrors. I bought a used set off ebay when one on mine developed a crack. The used set was very cheap and no one else bid on it. I did have to contour the mounting pad where it was the same curvature as the coach. I also got 2 pieces of stainless sheet metal that I rolled to the MH's contour on the inside. I cut a hole in the center of the sheet metal for the wires and drilled 4 holes in each for the mirror bolts.
So far it has held up for about 10,000 miles, and I like the larger mirrors and the fact they are chromed. The fiberglass in the front cap of my Beaver is pretty thick, yours likely is also.
Hopefully this will be helpful.