I have had the same problem. And a leak because the knife valve had been over tightned. For some reason where the cable is attached to the valve is on the far side of the valve, putting a sharp bend in the cable and making it very stiff.
This time when back at the dealer for repair. The dealer and I decided to rotate the valve 180 degrees and install a rod instead of a cable. they drilled a small hole thru the frame for the rod to go thru. It comes out right below the door where the cable handles are, you cant even see it without leaning down.
I dont know about your unit but the grey water cables are much more easy to pull as they dont have a severe bend. All I had to pay for were the extra plumbing fittings.
2012 305sk traded, and 2010 F-350 power stroke with 220,000 miles. 2012 F-250 power stroke
2015 Grand Design 337 RLS