Mike did a nosedive off a cliff that last week as well. Just like you describe, the treatment course was somewhat bearable. He had a feeding tube and continued to swallow and drink by mouth. Then Memorial weekend he crashed so hard I thought he might die. I found just sitting with him quietly helped. He tolerated very little but at least some of my attempts at the tube feeding, and would not swallow anything.
After that weekend his Radiation doctor precribed IV hydration for a few hours which helped a lot. He was allowed to get it whenever he wanted.

Ginny