I can understand your frustration.
I don't know that I buy the "bushings" diagnosis. Did they actually remove or uncover the motor and look at it?
I would try this, unplug the RV so it's running solely on battery.
Turn on some lights and then press the slide switch. Do the lights dim slightly?
Yes = Bushing thing is possible (still not likely though).
No = The motor isn't even getting power and the bushings aren't coming into play. Could be something as simple as a loose wire or bad switch.
Either way, if your not comfortable working on this yourself then I would have an independent shop or mobile RV tech work on it.
The $900 figure sounds like the dealer doesn't know whats wrong and is just making sure he makes money no matter what it needs.