Rita -
I would definitely call ahead to check availability, because besides evacuees, there are also many recovery workers who are staying in both Mississippi and Louisiana. I am currently in Baton Rouge, LA assisting in the Louisiana recovery efforts (have been here since November 1). Hotel rooms are definitely difficult to find, and many agencies (i.e. FEMA, Red Cross, etc.) are keeping the rooms reserved for their personnel. Many displaced families also still in hotels across the areas.
Good luck!