Seriously, you need to look at "The 12 volt side of life" and get yourself a voltmeter to see what you have in your batteries. You should be able to easily last for four days, but you might get a bit cold for lack of a furnace, and you won't be using TV or other nice things that you can have but really don't need for boondocking.
The best thing I ever did was get a meter, and check them every few hours to see what is going on, write it down, and don't let them get below 12.1 volts. You need a good meter that gives you voltage to two decimal points, so you can see when things go from 12.42 to 12.38 without just saying 12.4. You really need to chart it and learn how your batteries work for you.
