On our 2011 Sunseeker, the booster switch is on the left side plastic trim piece of the driver's seat, near the power seat switches. If I'm standing on the ground and open the door, it's visible. Sitting in the seat with the door closed, I can't see it. The switch has a car battery symbol on it. You press and hold it in to activate it. If you do it with the engine not running and the coach shore line not plugged in, you should hear a click when the battery control center relay is energized. If the engine is running or the shore line is plugged in, it may already be energized and nothing will happen when you press the button.
For your information, if you happen to run the house batteries down, you can use the same booster switch to use the chassis battery to supply 12 volts to the coach system so you can start the generator to recharge the house batteries. (The generator normally uses the house batteries to start.)
Or, you can just start the chassis engine, and in a short time, the battery control center will automatically connect the two systems, so the chassis alternator will supply the coach's 12 volt system. This is what happens when you are driving down the road, and the chassis alternator is supplying power to the coach system. This prevents the refrigerator and other 12 volt users from running the house batteries down while you are driving.
2011 Sunseeker 3100
1997 Pathfinder Toad