Opioids are generally well tolerated by dogs and would not be expected to cause skin disease.
I would wonder if there may be a secondary bacterial or yeast infection, which can occur in poorly dogs whose immune systems are under pressure.
We would also consider parasites.
I would advise a skin exam and swab as it may be that a course of e.g. antibiotics will help resolve things.
A short course of anti itch medicine e.g. Apoquel, may also offer relieve and allow the skin to heal.
Finally, a medicated wash such as Malaseb may help strip access bacteria and yeast from the fur.
All of these medicines would be available from the vet and should be well tolerated, even with kidney disease.
As it is affecting quality of life, I do think it is worth pursuing some tests and trialling treatment.
Hopefully we can offer some relief very soon as I'm sure this is frustrating for both you and her.