[Fulltiming]

Chester you are correct that to charge the engine batteries with the inverter/charger you turn on the merge switch. The reason it is an on-off-on switch is that one side fo the switch is hooked to the house batteries and one side to the engine batteries. With a single off-on switch if the batteries powering the solenoid were down the solenoid wouldn't work, if you turn the switch on and the meter for the engine batteries doesn't start increasing, turn the switch to the other on position and it will (unless the solenoid itself is damaged).