Well, far as I know, Tihama refers to the lowlands of the Hijaz. Since, the Prophet was HIjazi, he could be called Tihami also. It is definitely a title used for the Prophet and I don't think that there is anything wrong in keeping it as a name. Ahnaf and Tihami can also be combined, although if you insist on doing so, the best name would be Muhammad Tihami.