Apparently Tgrucelski has learned to love his mirror and not using it doesn't sound like something he wants to do.
Referring to the OP it sounds like the next thing to do is to contact Keystone and tell them what Burlington said about that model. It sounds like the persons at Keystone or Burlington generalized their answer instead of looking up the actual vendor used in the manufacture of the particular rig and get the vendor's name for the mirror. It starts with Keystone providing the correct information. Provide the dimensions (height, width, depth, whether it's convex or flat) and make them tell you who makes it (doesn't sound like Burlington unless the Keystone person can confirm). Then armed with that information try the vendor. A lot of times if the person you talk to can get you off the phone quickly then they are happy, not necessarily effective. Generalizations will do this. What you need are actual facts.
If you find you are not getting anywhere with them, contact an RV parts distributor giving them your specifications, or like Tom recommends try a mirror company. Or use the smaller mirror (unless you sent it back). It's possible with the quick response they did send you the wrong mirror - it happens. Lots of support calls when rushed will send the wrong stuff.
Whatever the resolution, we'd be interested in the outcome. I'll do a quick internet search and if I find something I'll post it. Good luck.
On edit: My internet search found that the Cougar model of trailer might have the size mirror you are looking for and maybe you can confirm the dimensions and use that as a replacement. This was patented by Keystone therefore they should have a replacement.
I also found these alternatives for a mirror at the front cap if you are unsuccessful at Keystone or their vendor. Note these are examples and you might be able to find better prices with a better search:
http://www.hitch-up-mirror.com/agora...htm#FifthWheel (removable magnet mount)
https://www.makariosrv.com/products/...5th-Wheel.html (requires drilling and attachment to the pin box)
Good luck to you