Again, this happened to me on my Cascade Eddie !
Tank was full , on the road, stressful. It appeared to be caused by corrosion between the cable and the sleeve. I used penetrating lubricant and sprayed it on the opening at the handle to the cable and sleeve and kept working it. I really had to tug on it but it did release. I cleaned the now apparent cable corrosion as best I could and then cleaned and lubricated it with grease several more times and so far so good.
Obviously this is pretty nasty to deal with if you are ready to dump as I was. Do all of this with the cap on and then deal with the cleanup with a hose.
I figured issue was due salt air from the environment where we spend a lot of time, but never had this issue with my prior camper.
Good Luck.