Carl,
I am with Fred in that I do not think that you can weld the broken mirror support (the cast piece that the mirror motors mount to). However if it is a Ramco mirror, you can contact their customer support to see if the mirror support is still a current production part and replace the broken part. I broke the mirror support on my current coach, and since the part was no longer made, I shipped my mirror head to Ramco and they rebuilt the mirror head with current production parts (new motors, glass, wiring, and support) for about $150. The only thing that they did not replace was the chrome plated plastic shell.
Gerald