I've really only had one experience like this, with a Rottweiler actually. She was on a raw diet, supplements, everything I could think of, but still that nasty coat. What I ended up using was a combination of a high quality shampoo/conditioner and Isle of Dog's royal jelly supplement. I washed her down REALLY thoroughly and then spent the next few weeks rubbing in the royal jelly every few days and brushing her down between applications of the supplement. Then at the end of the month I did another thorough wash and the royal jelly again. She shed horribly for a bit, but I think she may have been replacing dead, malnourished coat. After about two months, her coat was amazing, and actually a bit soft.
Depending on the dog, the effectiveness of this may vary, but that was the process that worked for me. After her coat was restored, I stopped using the royal jelly stuff but her coat stayed pretty much the same. Maybe not quite as soft because I wasn't using the fancy supplement, but it was still very healthy.