For *remote* - cable operated or electric black tank valves -
the best "fix" is to add a *manual* pull valve in the 3" drain line where it's *accessible*.
Cheap and easy to do......way, way easier than replacing (or lubricating) a remote valve up under the trailer!
Your bad back doesn't like bending to pull the valve?
Angle the pull handle upward for less bending.....and consider how your back will feel after laying under the trailer replacing the remote valve!..:(
Been there - done that.
If it fails - just as easy to replace again.
Then simply leave the (remote) cable or electric valve for the *black tank* in the open position.
Continue to use the remote gray water valve as you did before.
The remote gray water tanks valves just don't seem to fail like the black tank valves - maybe due to the waste - and/or the size of the valve.
(smaller valve operates easier.)
Happy dumping!
.