Landy, your car has a gear reduction type starter motor. I really doubt if it draws in excess of 85 amperes during the summer.
DAN, in 2 words "You can't" Only An Amp HOUR meter is capable of tracking long term variable discharge loads.
Back to Landy: Yep you're right. Your battery will last a long time. But with .105" plates I can suck my Lifeline down to 5% capacity using my BiPAP and get away with it time after time after time. I am not easy on batteries.
2,000 miles from here in Michoacan, my flooded cells are the same way. During a hurricane (we get those here as well) I draw those Rolls batteries down to 10% remaining capacity. Risking a pair of 100-meter dashes in a 130 kt wind is suicide for a person my age, so the generators stay in hibernation. Jesús, loses eight thousand dollars in Lobster defrosting, the Enramada goes belly-up and I just cannot bear the thought. By the way, a million dollar motorhome caught in a bad hurricane is going to leak water like a sieve. Quicksilver has no roof punctures and DuPont 5200 sealant around 5/16" thick tempered windows and it leaks.
I have a Cruising Equipment Co. AMP HOUR METER I refer to. But during a hurricane it's C-Rations, 3-watt LED lighting, and a fantastic Endless Breeze fan set on "1" Four giant refrigerator-freezers suck up power. i could no more "guess" at remaining capacity than a Tibetan Monk could manhandle a diving rod.