Sounds like you are describing a Velvac swinging-arm mirror, that's what most of the RV and box truck upfitters install on Ford and Chevy cab-chassis.
Ryder Fleet Products is one of the less expensive sources for replacements, but not repair parts. Theirs will likely be black and not have all the features usually put on the RV mirrors, so not an exact replacement.
Here is the tricky part. The mirror mounting changed 2003 E-series model year. So if your 2003 motorhome is on a 2003 chassis, you can use the 2003-2016 mirror. If on a 2002 chassis, you need the 1992-2002 mirror.