Redness of the scrotum doesn't sound like an STD to me either, although I understand your concern. In my experience, the best treatment for this is no treatment at all. If the ketoconazole doesn't make a difference in 2 weeks--and it probably won't--I'd just let it be and not pay attention to fluctuations in redness that come with heat, friction, etc. I think your doctors are correct.
Take care.
Dr. Rockoff