Hi David,
My husband is 2 year's post treatment and he still wakes up with the fluid under his chin sometimes. For him it helps if he sleeps with several pillows under his head. It's the lymphatic fluid and since the radiation frys everything, the fluid needs to find new pathways to travel. My husband massages his neck a lot which seems to help and he also has had several lymphatic drainage massages which have also helped. I would suggest you check into that.
Good luck,
Shelley