SaveRenderTexture.cs

using UnityEngine;
using System.Collections;
using System.IO;

public class SaveRenderTexture : MonoBehaviour {

    // Use this for initialization
    void Start () {
        Debug.Log ("'SaveRenderTexture' active. Press T to save the UI render texture.");
    }
    
    // Update is called once per frame
    void Update () {
        if (Input.GetKeyDown(KeyCode.T))
        {
            Save ();
        }
    }

    void Save() {
        Camera cam = GameObject.Find ("UICamera").GetComponent<Camera> ();
        RenderTexture renderTex = cam.targetTexture;
        RenderTexture.active = renderTex;
        Texture2D tex = new Texture2D (renderTex.width, renderTex.height, TextureFormat.ARGB32, false);
        tex.ReadPixels (new Rect (0, 0, renderTex.width, renderTex.height), 0, 0);
        tex.Apply ();
        RenderTexture.active = null;

        byte[] data = tex.EncodeToPNG ();
        File.WriteAllBytes("renderTexture.png", data);
        Debug.Log ("Saved texture to: 'renderTexture.png'");
    }
}