Worried Daughter,
I thought the first two weeks after radiation were the toughest and I went back to work only to find out two months later that my energy was just totally drained. I checked the thryoid regularly but just got totally exhausted. Everyone is different, though.
I had a PEG tube but I rarely used it except for Robitussin cough syrup to thin out the thick goo in my throat. That stuff will make your hair stand up if you drink it. It was like liquid fire, to me

! I could not get more than about half a can of Ensure down the tube and it would come right out the mouth. The doctors determined my stomach was positioned wierd and somewhat smaller so they tried to let some of the water out of the bulb inside but to no avail. It was more hassle and hurt a lot of the time but they wouldn't let me take it out for almost 8 months. Again, that was just my case and not the norm.
Remember caffeine in tea will increase the dehydration and de-caff would probably be a better choice.
Your mother will get better, especially with so much love around here. I wish you all the best!
Ed