Now that we've gotten that out of the way, deal with the principles.
Feeding tubes are no longer an extraordinary means of preserving life. This is not 1958.
"Having a hole poked in you, a tube shoved in and then having to eat and drink that way would be burdensome for any normal man."
Burdensome schmurdensome!!! If that were the case then one could argue that the millions of people who use colostomy bags have a license to commit suicide as well, on the basis that they have to put up with holes being poked in them and tubes "shoved in".
Provision of nutrition and hydration, whatever the means, cannot be considered "extraordinary measures". These are basic requirements for life.
I fully agree with the comments made by sinkspur and gbcdoj on this one. If your approach to moral questions is reduced to "can I get away with this without committing a mortal sin?", then sinkspur is quite right - it is casuistry, and it stinks.
Even to ask the question "Is this 'mortally' sinful?" is an admission that the proposed course of action is wrong. To set out with the deliberate intent to commit a sin, particularly where the loss of a human life is the result of that sin, can itself be a mortal sin irrespective of the gravity of the specific action which is proposed.
If any Catholic, or other Christian for that matter, is serious about serving God, they do not ask "How far can I sin without losing my soul?", they ask "How can I do what is right and best and true?".
Amsterdam eagerly awaits your presence.