Without seeing the rash I cannot be sure, but it sounds as though you are describing the skin changes which come with chronic sun exposure. It's hard to imagine that your dermatologist can't tell what this is by looking and performing a biopsy. Rosacea doesn't affect the neck--just the nose and mid-face. Perhpas you should get another opinion. It may be that the only treatments available are cosmetic ones like the use of vascular lasers.
Best.
Dr. Rockoff