Oh gosh, OP. This is so difficult. I have no advice really. Only sympathy.
I think, as adult-children, especially those without siblings to help out, we feel a tremendous burden of obligation towards our parents.
My dad died three years ago nearly, and was very ill in the year before he passed. I had such huge guilt that I couldn't be there every day to help care for him. I lived 2 hours away and had a full time primary school teacher job. The stress and the guilt was incredible.
What helped me a bit was to ask myself "what would 25 years ago dad have said to do?" He was fiercely protective of his family, and only wanted us to be happy and safe. And I feel like he'd have said something like "don't worry about me, live your life."
OP, would your dad have said something similar? Would he want you to financially ruin yourself for the sake of a few weeks/months together? He wouldn't want you to have this tremendous guilt and obligation.
As it happens, in my case, my dad was actually doing much better, and was seemingly almost recovered when, early one Saturday morning, he just quietly slipped away in his sleep. He was only 62, but in many ways, I was grateful that he'd found some peace.