This isn't what you want to hear but no one on line can tell YOU what YOU will get as an average fuel consumption on YOUR car.
No one knows how you drive, where you drive or the kind of journeys you make - short or long - all of which will have a bearing on fuel consumption.
If you get 17 mpg over 15,000 miles per annum, you will have to buy 880 gallons of fuel
If you get 23 mpg over those same 15,000 miles, you'll buy only 650 gallons.
If the cost of buying, in the worst case, those extra 230 gallons each year, is really important in your family budgeting, then to be very blunt, you should not be buying a Touareg.
These cars are not cheap to run and not cheap to maintain.
And given what you have already said, you also need to factor in the cost of a very good warranty so you don't get unexpected and expensive repair bills.