There are RV valve lube products that may help. I use Thetford Drain Valve Lubricant after dumping the tanks. Both grey and black.
Try to get a penetrating oil like WD40 into the cable. A motorcycle cable lube tool might work...I've never tried. Some folks have drilled a hole into the valve housing to get lube into it. Fortunately I've never bad a valve stick bad enough to try this approach.
Worse case, disassemble, clean, lube, reassemble.
2017 Riverstone 38FB Legacy