Service manuals are still available for the 1973 Airstream. I did a Google search and found a number of listing, such as this one.
1973 Airstream Factory Service Manual
The service manuals for the 70's model trailers provide detailed information and schismatics of the all systems. When I bough my 1976 31' Sovereign
it had been sitting up for over 10 years like yours was. I found the service manual stuffed in a garbage bag with other trash and I am really glad that no one got around to throwing it out before I bought it. It has been a real help in my continuing refurbishment over the past 12 years.
Good luck with your project. Post some pictures, we love to see pictures.
1976 31" Sovereign