Dave- Older coaches, and cheap coaches even today have a "manual" xfer switch that is actuated by inserting the shore power cord into a receptacle fed by the gen. To get shore power you unplug from the gen & plug to shore.
Alpines & most current coaches have auto-xfer which senses where the feed comes from & switches accordingly. You'll hear a thud when the switch moves sources. If it becomes a thud-thud-thud-thud, either the power source is unstable (e.g. shore plug nearly in contact and arcing) or auto-xfer is busted.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project