I was thinking exactly the same as "J.Morgan."
I would be inclined to give the threads a shot of penetrating oil, leave it a while then have at it with vicegrips on the valve stem in order to be able to use up the propane, the when empty, take to a dealer for a new valve.
If the vicegrips didn't do the trick I would just take it to the dealer "as is" to deal with!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.