The Tradewind and the Globetrotter appear to have the save toilet
configuration. Look at the 63 catalog at
http://airstream.net/members/documents/catalogs/
So I would expect the tank to have been in that raised platform.
You are right in that the gray water lines do not drain into the black
tank .
I would probably search thru Tom Patterson's archive to see what has
been discussed before about replacing black water tanks and also look at
some of the used parts dealers to find a tank.
I did get a new fresh water tank from All Rite (www.all-rite.com). They
say they can make custom tanks.
Herb