help needed trying to save render texture as png

I have two cameras.

1 - main camera which rotates around my scene and shows the GUI.

2 - small thumbnail camera in a fixed position doesn't show UI, and has it's target set to a render texture.

Camera #2 works perfectly as a picture-in-picture camera, but I can't figure out how to save the render texture to a png; using this camera's position and rotation, etc.

The code I have (below) is attached to my thumbnail (#2) camera and it will render a png, however it's not what camera #2 sees, it's whatever the main camera (#1) sees; which is a completely different point-of-view and it includes the UI.

I simply want to read the pixels of the render texture (which is assigned to thumbnail #2 camera and it looks perfect) and write them out. I don't understand what I'm doing wrong, or maybe this isn't possible.

Here's the code I've pieced together from stuff found on the web:

     public IEnumerator TakeScreenShot()
     {
         int startPosX = (Screen.width - slotTexture.width) / 2;
         int startPosY = (Screen.height - slotTexture.height) / 2;
 
         Camera camOV = OVcamera.GetComponent<Camera>();
 
         Texture2D imageOverview = new Texture2D(camOV.targetTexture.width, camOV.targetTexture.height);
 
         RenderTexture currentRT = RenderTexture.active;
         
         RenderTexture.active = camOV.targetTexture;
 
         camOV.Render();
         
         yield return new WaitForEndOfFrame();
 
         imageOverview.ReadPixels(new Rect(startPosX, startPosY, slotTexture.width, slotTexture.height), 0, 0);
         imageOverview.Apply();
 
         RenderTexture.active = currentRT;
         
         // Encode texture into PNG
         byte[] bytes = imageOverview.EncodeToPNG();
         
         path = Application.persistentDataPath + "/" + filename;
         
         System.IO.File.WriteAllBytes(path, bytes);
     }
 

Anyone have an idea how this can be done?
Seems like I should be able to have several cameras (in various locations) each with render textures assigned and then save them out.

Help!