The 40# tanks on my 1972 Airstream Land Yacht are period appropriate, which is to say, they are original to the Airstream! It would seem propane valves have changed over the years. The interesting thing about my valves, is that they are able to take multiple connections.
Unfortunately, the gasket on the inside of the valve is shot. On modern connections (like the connection on a grill), the hose connects to large threads on the outside of the valve. This connection seals when the male part of the piece you screw on presses up against the back of the female part, causing a seal at the aforementioned gasket.
Evidently you can’t just replace the gasket. The place I have certify and fill my tanks searched and searched, but they were not able to acquire the piece inside the valve that has the failing gasket. Thus, you have to replace the entire valve. Replacing the valve is around $55. My valve, however, also has internal threads, and there is an adapter that will go from those internal threads to the large external threads we have today. That adapter is $21. The adapter is configured so that it presses against the gasket that has failed in my valve, but since it also threads into that stem, I can put gas teflon tape (the yellow stuff) on the threads to seal up the connection.
Thus, this adapter is a workaround for making sure my vintage valves are safe and sealed when using a modern propane connection.
Here’s another post on BeahmStream.com with more information regarding RV propane tanks.