Don't discount the lights as minor consumption. Each incandescent bulb is a min of 1 amp. That can add up quick.
Another thing - if you are a TV watcher - get a 12v native LED tv set. Running an inverter to power an AC TV is also consuming alot of amps unnecessarily.
I'd agree with above, if refer on gas, LED lights and TV, a little pump use, you should have no problems overnight even if using the furnace. Without the furnace, a whole weekend should not be a problem. Any use of an inverter for things like a computer, or for any appliances, then 1 battery is not enough.
Besides CO, I'd be concerned with heat buildup in the exhaust system with excessive idling. So be cautious about what is underneath your vehicle (like tall grass)if you are going to do it.