Agree, if your house batteries are depleted, they will likely take 6 to 8 hours of generator time to recharge to about 80 or 90%. If you keep running them down to 11 dcv, you will shorten the useful life of the batteries. Best to begin recharging at about 12.0 dcv which is about 50% depleted.
Inverter on wih an icemaker or water heater on te inverter circuit would draw them down rapidly. Another big draw are the propane furnaces.
Full charge is about 12.6dcv. Stop the generator, turn on some house lights for about 1 minute. Then apply the leads of a digital multimeter to the house battery terminals. They need to read at least 12.6v to be near a full charge.
You can buy a meter at auto or hardware stores for about $10.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs