How To Propane valve stuck in closed position

[vhuerta800]

I have a 1990 fleetwood tioga montara and the open and close valve is stuck on the propane tank. It's 3/4 of the way full right now and I've tried to use pliers and spray a little grease on the threading in hopes that it'll come loose. Any help is appreciated. Also had my roommate try to open it too.