Thanks all!
It is indeed a Thetford valve. Says right on it.
Now that the black tank is drained (another long story) I can open the valve and see what's going on. The rubber seal is all deformed - it's toast. It appears as if the blade is plastic.
It would be cool if there was a Thetford-to-normal-bayonet adapter but I haven't been able to come up with one. Maybe hacking that one out is an option.
If I replace the valve with a newer style, I'd have to hack the elbow off at the tank, which might not be the end of the world if I could slip a new elbow over the remnants. I'll have to hack away all that foam insulation to see what I'm dealing with.
In the meantime, I'll look for a thetford rebuild kit. Might be the best short term solution.
