Look at Vintage Trailer's web site. They have a cast aluminum water fill for $65 that might work for you. They also have tanks and other styles of water fill systems. Good company we have ordered from many times.
This website is not affiliated with or endorsed by the Airstream, Inc. or any of its affiliates. Airstream is a registered trademark of Airstream Inc. All rights reserved. Airstream trademark used under license to Social Knowledge LLC.