First of all, prayers for your dad and your entire family.
Secondly, you are making the right decisions if there is any chance of recovery, no matter how remote. God bless you for your decisions, and please do not let others sway your Catholic convictions. (I don't think you will, but my prayers are with you.)
When my husband died, he had decided to stop the radiology treatments himself. It was difficult for me to accept this and not feel abandoned by both him and God.
May God continue to walk with you and your family during this difficult time.
Salvation, it is very sad that you faced that ordeal in losing your husband to cancer, and it is a situation which so many people face. No doubt, when he ended the medical treatments, he was making a necessary decision, for there comes a time, for all of us, when earthly help is of no avail. But it is so hard for the family.
AAABest, I will be praying for you. As for advice, I suggest that you see your father, and pray at his beside. The decision might come naturally then. Don't let anyone insist on things, or bully or rush you, you will know what to do at the right time.