Methdozer is correct, when you are plugged into 120 power after running off 12 volt for a while it will charge your batteries as well as supply 12v power. As well, even if your batteries are full and you are plugged in to 120v using lights (12v) furnace fan (12v) water pump (12v) your inverter will supply the 12v and fan will kick on to cool under heavy load. If your rig doesn’t have its lighting changed to led bulbs yet, do so. Takes a major draw off the system in general. I converted all my ceiling lights to led bulbs and when plugged in (or running genny) cooling fan rarely turns on now. Installing a solar panel on the roof helps as well (I went with a 150watt)
