Don’t sign the advance directive unless you don’t mind being starved and dehydrated to death. My advance directive now states as follows: I desire to be fed and hydrated.
The essential thing is to have a Health Care Proxy who knows quite thoroughly what you would find necessary or unnecessary, tolerable or intolerable. The "piece of paper" is indispensible because it provides evidence. But even the "piece of paper" isn't going to save you, if you don't have a living person there to to actively and intelligently advocate for you.
That's why I recommend the NRLC "Will to Live" document.
You can put that into an advance directive. All it does is tell the world what you want done to you if you are incapable of telling them yourself.
You can say keep me on a machine till my flesh rots of my bones if that’s what you want.
Life is seldom as simple as it looks.
If you have ever watched someone taking their last gasps of air dying of cancer, you would realize that the quiet end that comes with dehydration is not the worst fate.
Everybody needs to make their own choices but if my end were cancer I would choose to not receive a gastric tube or IV fluids.