1993 Hymer B564 External Gas Fitting

Hi Phil,
I would Email the main Hymer dealer in Germany with the photo, VIN, Model of yours and there should be no problem with providing the Replacement Part number, price and details of your local Hymer Dealer.
The afterservice from Germany is second to none although it takes a week or two for a response, you get an informed response and a viable replacement if original part not available, not a "this will do".
I use Internet Explorer with Word and the translation function where i email in English and IE's German translation - God knows what it is saying - but i haven't got the door slammed yet
I replaced my BBQ attachment on Hessy with one i got local and used a bit of sandpaper and PTFE Tape to make fit. Tested leakage with turning on gas and applying flame, tightened till any leaking gas disappeared. As outside and no build up of gas possible the only risk was burning Hessy's plastic BBQ outlet.
Could you not wrap-up the valve in plastic film and fill with olive oil or glycerine, let it soak for a day or 4 and then try to move easily in any direction. They are brass fittings from this era and usually vaseline or similar is used for the lubricating medium - so corrosion at the gas control end shouldn't be that much?
Hope you get it working