SaveRenderTextureToFile.cs

using UnityEngine;
using UnityEditor;

public class SaveRenderTextureToFile {
    [MenuItem("Assets/Save RenderTexture to file")]
    public static void SaveRTToFile()
    {
        RenderTexture rt = Selection.activeObject as RenderTexture;

        Texture2D tex = new Texture2D(rt.width, rt.height, TextureFormat.RGB24, false);
        RenderTexture tmp = RenderTexture.GetTemporary(tex.width,tex.height,0,RenderTextureFormat.ARGB32);
        Graphics.Blit(rt,tmp);
         RenderTexture.active = tmp;
        tex.ReadPixels(new Rect(0, 0, rt.width, rt.height), 0, 0);
        RenderTexture.active = null;

        byte[] bytes;
        bytes = tex.EncodeToPNG();
        
        string path = AssetDatabase.GetAssetPath(rt) + ".png";
        System.IO.File.WriteAllBytes(path, bytes);
        AssetDatabase.ImportAsset(path);
        Debug.Log("Saved to " + path);
    }

    [MenuItem("Assets/Save RenderTexture to file", true)]
    public static bool SaveRTToFileValidation()
    {
        return Selection.activeObject is RenderTexture;
    }
}