Severe Itchy Red Rash Over Body

My 87 year old, otherwise healthy, mother has severe itching with a red rash for years that comes and goes but is worse now more than before. She cannot sleep without waking up itching and scratches her thin skin in her sleep causing the blood vessels to break and bleed under her skin. This causes unsightly black bloody bruising. Topical creams OTC or prescription do not help. Blood work is normal. She has tried ever elimination diet from Salycilate free to Gluten free with no relief. She does have Rhematoid Arthritis. The muscles under the skin is very sore. The rash covers her back,chest,arms and legs but not her face. Dermatologist, RA doc and GP all scratching their head as to what is the etiology of this extremely severe itch and red rash.