Cable is frayed so it sounds like the cable has come completely detached from valve.
You need to gain access to the valve.....if you have an enclosed underbelly then you can remove a panel OR cut a three side window (non-cut side towards front of rig) so that you can get to the valve. Then you will see IF you have any options of reattaching cable, replacing cable or if you will have to replace the valve assembly.
Then IF you have to replace valve assembly you will need to empty tank....either a septic pump company or pry valve open via stem at a dump site (pliers, screwdriver, hack saw---whatever it takes to get at valve stem so you can force it open)