Your dump valve is an older style Thetford which is no longer available. Airstream stopped using it in 1964
. If the brass (not cast aluminum) is in good shape, you might think about rebuilding it by replacing the 3 internal rubber seals.
As you have probably noticed, modern fittings won't fit on this valve. If you don't have the old hose connections which fit this valve, you'll probably want to replace the whole valve with a modern one.