While it is probably possible to add a larger fresh water tank to any Airstream, Airstream itself does not offer such an option and the cost of retrofitting an Airstream on which you are not already doing a frame off restoration would probably be unattractive.
We boondock most of the time (over 200 nights per year) and also have a 39 gallon fresh water tank in our Airstream. We also carry six five gallon plastic jerry cans (BPA free.) We will fill the jerry cans when we know we are going to be away from an accessible water supply and can make it seven to eight days with our 69 gallons (two experienced adults, no kids.) If necessary, we can make a run into town to get more fresh water in 30 gallon increments using the jerry cans which is good for about three to four more days.
The black tank usually fills up in twelve to fourteen days which is usually what makes us break camp.