I think I can say that for most of us, it is normal, unfortunately.
Are you sleeping with you mouth open? I find that I do sometimes and that really adds to the effect.
Some here have suggested wearing some sort of chin strap to keep that sucker closed.
I am 13 months post IMRT and still have both the Biotene and Water bottle on the bedstand. It is not as bad as it used to be so yes it does get some better for most.
Kevin