a cable is a cable.
I'd suggest you call around to a motorcycle shop ( the older the better) and ask if they make up cables ( like throttle/ clutch)- all they have to do is match the size of teh ferrules on the ends.
matter of fact home depot has cable ( just the inner) and ferrules.
the hood release cable on my pick up broke about 10 years ago- I replaced the wire by threading a D bass string ( .065) into the sleeve and crimping a ferrule at the correct length.
here's a link to valterra, they sell cables
and their products are available all over the web, cheap
I would expect that a cable actuated valve uses a screw in terminal which would accept a regular pull handle, so you can open teh valve if need be.
not sure where you are but I found Country Roads RV in Florence OR to be VERY helpful