Dermatitis and eczema are somewhat overlapping. From all the medical stuff I've seen the biggest difference is in the pathology of the skin. The sebum is basically consumed by yeast, infection and inflammation result.
Eczema, however is more of an umbrella term that many use to cover a variety of dry skin conditions. Sometimes seborrheic dermatitis get's included under this term as well.
In terms of the steroid use. I personally believe that they are no good for seborrheic dermatitis. Even though they do control it initially, they are not suited for long term use. In addition to this, long term use has been associated with development of rosacea like dermatitis (source).
What I would do is try to find a local clinic that carries a good variety of samples. Some people react really well to some of these. For me the Restoraderm seemed to work exceptionally well.
For food you experience this from absolutely everything? And have you tried any sort of dietary restrictions to see if you might have some sort of allergy to something?
Consider increasing cruciferous vegetables in your diet. They have documented benefits on improving cellular health and improving bodily peptide production.
If your condition is limited to strictly around the mouth, it seems like it may not be seborrheic dermatitis. SD typically presents itself in more sebum reach areas of heat. Perhaps it maybe some sort of fungal infection, which will likely require a corresponding lotion (nystatin, ketocanazole, UVB radiation, zinc pyrithione). I would personally try to see if it responds to a zinc oxide based diaper rash cream (like SudoCrem).
Hope that helps and your problem goes away! Let me know if you need more information or if I have missed anything.