I had a similar issue on the grey tank valve. I removed the valve & seals, cleaned them and reinstalled them......problem fixed. There was no apparent "gunk" anywhere that would have caused the problem. The only thing I can figure is that when the valve was installed at the factory, the 4 bolts were not tightened to the same level and the valve was binding. When I reinstalled it, I tightened the bolts slowly checking the operation as I proceeded. I also tightened them in a crossing pattern like you would do on a wheel - tighten one then do the one that is directly opposite it.
Hope this helps!
Merry Christmas and Happy New year to all!