They really could be a lot of different things. It could be various types of evergreen oaks. A number of evergreen Quercus species are hardy in southwest parts of the state. A few different types of magnolias, but especially Magnolia grandiflora (southern magnolia) and some Magnolia virginiana (sweet bay magnolia), are evergreen and hardy in Tennessee. Various camellias can get to over 20' tall. Some types of Rhododendron can get to over 30' tall. We really need a picture or two (especially a close up of the leaves) to make a good ID.