Back in the day when electric cigarette lighters in a car were actually used for lighting cigarettes you could draw 30A or sometimes more from most any outlet. Wire gauge in the dash and fuse rating was plenty for drawing that much current.
Now they’ve become neutered “accessory” power outlets for cell phones. I was recently surprised my iCharger had a helluva time drawing more than 10-12A from a 2008 Escape/Mariner outlet.
At one point blew the **** 15A fuse. That’s not gonna supply much power to something like an DC/AC inverter or some other type high power device.
Another issue, unless you measure Ah/Wh consumption using inline power meter it’s awfully hard and/or impossible to “guesstimate” how much energy’s taken from a vehicle starting/cranking battery.
Key description is: “starting/cranking” - supplying battery power over a long period of time is much different than momentarily spinning an IC engine for starting. You’re not doing your vehicle starting battery any favors running loads without the engine running. And then you're not doing your vehicle or MPG any favors idling for long periods of time.
For me at least in the field, ‘now bring a separate 12V battery to power my invertor or RC Charger. Preferably, deep cycle or something along those lines.