Have you tried tapping it with hammer? Boiling water?
Blow torch should do something
Have one like that on our holding tank. Same design, different label. Also was seized up real good. Soaked it with Fast Release Spray from WD-40 for a few days, then found a piece of 4mm flat bar that needed only minor filing to fit snugly in the slot. Two big adjustable spanners on that and a piece of timber across the top. Then leaning over the thing, pushing the timber and flat bar down with my chest while turning both spanners with full force. A few attempts and it started to turn. Now with new o-ring and silicone grease on it, turns much easier.
The flimsy stamped metal keys you get for these are a joke and bend like wax. I unbent mine and returned it for a refund
If you need a new one, don't buy it from Carver, it's a generic marine part, plenty available on the Internet. You'll get ripped off slightly less that way.
A good cleanup after you have got it off and some copperslip on the threads for next time.