Her mane and tail are always going to feel coarser than her body hair.
As for her coat, a couple of things to consider:
1. Are you feeding the manufacturers recommending feeding amount by weight? The nutrient balance of a complete feed is only truly effective if fed in the appropriate amount.
2. Are you feeding with grass hay or alfalfa? The Blue Seal (this is what I'm assuming is the brand name of the feed, although it appears the name has changed to Sentinel Senior) website says this feed is meant to be fed with a grass hay base.
3. Between the senior feed and your hay/pasture, the diet may still be deficient in something, so testing the hay/pasture, or getting a blood test done to determine if there is a deficiency may help.
4. If you like how your horse is doing otherwise, an addition of flaxseed to the horses ration may help.
5. ETA, one last thing, some horses may just have a coarser feeling coat than others, just like some humans have coarse hair, while others have fine hair.