Since the mane and tail used to be in better condition, there must be some cause for the dryness. If you can rule out dietary issues, you can try massaging mineral oil or baby oil into the skin along the crest and tail. This worked for one "dandruffy" gelding. If the rest of her coat is shiny and healthy, a feed through product or coat supplement probably won't do you much good, but you could try it.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown