There should be a transfer switch that switches from generator to shore power when you plug in. It is under the bed in our coach.
The battery isolator is located in a box beside the house batteries. After a delay of some minutes the chassis battery is connected to the house batteries via a solenoid in this box. It works the same way in reverse when you are running the generator or are on shore power so it keeps the chassis battery charged while the rig sits idle.
Both of these things get cranky when they get old. Especially the battery isolator because it gets switched so often. The contacts get burned and pitted.
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