Hi, Brendan
My husband used a humidifier, and slept propped up for the worst of the dry mouth/phlegm duo. He kept a water bottle by the bed. Some of the Biotene products for dry mouth may help. If you have any nasal congestion, talk to your ENT about a nasal spray to help - if you are not breathing through your mouth, it is a little easier. Do not be hesitant to discuss all this with your ENT, RO or MO! The better (or less wretched) you feel, the better you will be able to get through the treatment tunnel.
The fact that you are fit is going to help you recover when the TX is done. Hang in there!