That cable, if it's a Valterra Bladex valve as mine were, the cable can break at the valve. Instead of having a set screw to hold the cable on the valve operator rod like the replacement Valterras, is simply bent over in the rod. This bend is a very sharp 90 plus degree bend which will put a breaking point on that cable. Also, the way the outer housing is held is suspect in that it is a nut device that squeezes the outer valve body tight against the housing. The pull handle will tell you if yours are Valterra
I installed new Valterra valves and cables* last week
https://www.amazon.com/Valterra-TC37...e%2C228&sr=1-6
I'm guessing that you will need to drop the belly cover and operate the valve manually, using a pair of Vise Grips. You can run a hose back from the outlet end but if indeed it's a clog, you will have a mess on your hands. If you do give that a try, I suggest a Valterra twist on waste valve** on the end of the line so if you do break a clog loose, you can limit the mess
**
https://www.amazon.com/Valterra-T58-.../dp/B000BGHYJS
Good luck - and let us know what you find