Prior to 1973, Airstreams did not have gray water tanks. If the rig that you are looking at has the below floor black water tank, it is possible to back-up the gray water into the black water tank by placing a solid terminator cap on the dump valve and opening the dump valve allowing the gray water to back-up into the black tank. (I know the 1960
model Airstreams that I have seen have the black tank above floor level while my '64 has the tank below floor level - - it is my understanding the 1964
was the first year for the below-floor tank for most Airstream models. The down-side to this method is that when you go to dump the contents of the holding tank (first be sure to close the dump valve), there will be about 1/2 gallon of sewage between the now closed dump valve and the terminator cap which must be caught in a pail to be disposed of before the dumping hose is attached. That is why the owner of the rig you are looking at suggested the portable tank - - it isn't the greatest solution going, but it will keep the rig legal in most parks, I have been using a portable blue tank with my Overlander for the past eight years - - choosing one size that will work best for you might be the most difficult decision - - I have two - - one is 20 gallons and the other is 10 gallons and each is on wheels for easier use.
I have considered having a gray water tank added, but it poses the problem of adding additional weight behind the rear axle which isn't terribly desirable. It is also rather expensive when done by the professionals. My Airstream dealer estimated the cost of installation and adaptation to be around $1,200.