It will take about 2 hours in the morning and two in the evening using the generator and your onboard charger. Do not try to use the small charging cable from the generator if it has one. They often put out only 8-10 amps.
If your generator has the ecomode it will run on high and you can listen to it go to the slow mode as the batteries get charged.
If your lights inside are not led and you have an inverter for entertainment devices you could be stretching two batteries to their limits assuming they are in decent contition to begin with.
Do not rely on some led panel. You need to get a voltmeter on the batteries before and after as suggested.
A furnace can draw 6-8 amps and drag down batteries quickly if it is cold.