Nobody said you were looking for a free lunch.
Go to http://en.wikipedia.org/wiki/Peukert%27s_Law
and down at the bottom click on the link to Gibsons Excel spreadsheet for calculating usage.
A 200AH battery (20 hour rating) when discharged at a 15A rate will last 11.81 hours. 10A will go 20 hours. It doesn't say if that's fully discharged or half.
Now how much current do you want to draw and for how long? From that you can calculate the battery size needed. I would suggest putting an ammeter in line and measure the actual draw with normal operation as you plan it.
I would guess and say based on the heater drawing 6.7A max but not on all the time plus the current for the fridge (even on propane it needs current to light the gas) plus a couple of lights you could go at least one night (12 hours) on a 200AH battery setup. If you drive your vehicle every day and it's setup to charge the batteries you should be OK. All of that is a guess.