We are obviously doing it wrong at the moment. For our 3 month trip from Vic to Kakadu, Darwin, Kimberley, and the Tanami, we had a 50l National Luna (40l fridge and 10l freezer) and did it easy. Left home with some vacuum packed food and topped it up with fresh or sealed as we hit the bigger towns. Always have a dozen stubbies at the bottom of the fridge where they are cold (I like that too) and top it up as a six pack is used. I suppose it really depends on how often you are going to hit towns, with the chance to re-supply. We even carried a box of magnum ice-creams (a weakness of mine) and really didn't wish for more space overall. In fact near the end of the trip I was filling up plastic bottles of water and putting them in the fridge and freezer just to fill them a bit and save what was left from bashing itself to death.
The weight of 3 full fridges must be fairly high, and I would like to see a comparison of buying food in a capital city and then paying the fuel costs to take it on a long holiday, as opposed to the slight increase in price of the food purchased inland.
Just my thoughts.