EDIT 1 Here is how I apply uv

` public static Dictionary<string, Vector2[]> textureMap = new Dictionary<string, Vector2[]>();

public static void Initialize(string texturePath, Texture texture)
{
    Sprite[] sprites = Resources.LoadAll<Sprite>(texturePath);

    foreach (Sprite s in sprites)
    {
        Vector2[] uvs = new Vector2[4];
        uvs[0] = new Vector2(s.rect.xMin / texture.width, s.rect.yMin / texture.height);
        uvs[1] = new Vector2(s.rect.xMax / texture.width, s.rect.yMin / texture.height);
        uvs[2] = new Vector2(s.rect.xMin / texture.width, s.rect.yMax / texture.height);
        uvs[3] = new Vector2(s.rect.xMax / texture.width, s.rect.yMax / texture.height);
        //Debug.Log(s.name);

        if (!textureMap.ContainsKey(s.name))
        {
            textureMap.Add(s.name, uvs);
        }
    }

}

public static bool AddTextures (Block block, Direction direction, int index, Vector2[] uvs)
{
    string key = FastGetKey(block, direction);

    Vector2[] text;
    if (textureMap.TryGetValue(key, out text))
    {
        //Debug.Log(key);
        uvs[index] = text[0];
        uvs[index + 1] = text[1];
        uvs[index + 2] = text[2];
        uvs[index + 3] = text[3];
        return true;

    }

    text = textureMap["Default"];

    uvs[index] = text[0];
    uvs[index + 1] = text[1];
    uvs[index + 2] = text[2];
    uvs[index + 3] = text[3];

    return false;
}

`

Where Direction is a direction (example north) and Block is a short. Index is the vertex index of the mesh. Texture is the sprite texture (.png)