There could be one of a few things wrong.
The worst would be that the valve blade is jammed in the valve and the pull of the handle to dump pulled the metal handle out of the valve entirely. This is a worst case scenario.
The more likely issue is a matter of the handle disconnecting from the rod that attaches to the valve. In either case you are going to have to get access to the valve assembly to fix it. Look for an access plate on the belly skin, straight in from the pull handle. Airstream uses an extension rod in many cases to get to the valves. This rod is normally connected to the valve with a cotter pin or a rivet. If the connector has broken, it is a simple matter to replace it. The valve could also have a leak around the rod o-ring. This could contribute to rust that will make the valve hard to move. This stiffness can lead to the failure of the connector. Once you have the valve open, inspect the rod for rust. If it is present I would clean with Emery cloth and lube it liberally with silicone spray. Run it in and out quite a few times to get the O ring lubed. This may be all it takes. It may take more.
If the leak is bad you could have to replace the whole assembly. If it is minimal you could use it as is. If the rod will not pull out, twist it in place. It is designed to be twisted. The twisting motion will help break it free of the rust.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato