I'm having a similar problem with the black valve on my 5th wheel... BUT... the trailer is 13 years old and so is the valve. And I've lived in it fulltime for 12 of those 13 years. This started earlier this year and is getting worse. I can still get it open but it's very difficult and I'm worried about breaking it.
I've torn into the underside of the trailer, and I can replace it, but just have to get around to DOING that. Until then, I love the ideas posted here about lubing the cable. I'm going to try those.