[Fulltiming]

The system is set up to isolate the engine batteries from the house batteries. That way, if you are using the inverter to power things inside the coach you are assured that the engine batteries will not be discharged as well. The engine batteries are charged from the engine alternator. Not sure when, or if Newell changed it, but at least during the early 1990's, starting the engine automatically merged the engine and house batteries to allow all of them to be charged off the engine alternator. The battery charger portion of the inverter charges just the house batteries UNLESS 1) you have the battery merge switch on the dash turned on or 2) your coach has the echo charger (or a similar aftermarket device) which feeds current from the house batteries to the engine batteries anytime the house batteries are being charged and the engine batteries are lower voltage than the house batteries.

Upside of keeping the merge switch on when the coach is plugged in to shore power: engine batteries, as well as house batteries stay charged. Downside: if power goes off or a house battery develops an internal short, the engine batteries, as well as the house batteries will drain and you will not be able to start the coach.