A Unity Editor Window for rendering a camera you reference directly. Works with cameras in Prefab Staging scene. Put inside an "Editor" folder, like Assets/Editor.

DirectCameraWindow.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEditor;
using UnityEditor.Experimental.SceneManagement;
using UnityEngine;
using UnityEngine.SceneManagement;

//drawing a camera in gui:
//https://answers.unity.com/questions/312472/how-to-change-size-of-camera-preview-in-editor.html

//getting screen res:
//https://answers.unity.com/questions/560518/get-game-view-window-current-width-height-in-scrip.html

namespace Turnfollow.Utilities
{
    public class DirectCameraWindow : EditorWindow
    {
        private Camera _camera;
        private Vector2 _cameraRenderSize = new Vector2(1080, 1920);
        
        //set these to skybox if you use skybox
        private CameraClearFlags _clearFlags = CameraClearFlags.SolidColor;
        private RenderTexture _renderTexture;
        
        private const float CONTROL_AREA_HEIGHT = 80;
        
        [MenuItem("Turnfollow/Direct Camera Window")]
        private static void Init()
        {
            DirectCameraWindow window = GetWindow<DirectCameraWindow>();
            
            window.titleContent = new GUIContent("Direct Camera Window");
            window.autoRepaintOnSceneChange = true;
            
            window.Populate();
            window.Show();
        }

        private void Populate()
        {
            RefreshRenderSize();
            RefreshTexture();
        }

        private void RefreshRenderSize()
        {
            string[] screenRes = UnityStats.screenRes.Split('x');
            _cameraRenderSize = new Vector2(int.Parse(screenRes[0]), int.Parse(screenRes[1]));
        }

        private void RefreshTexture()
        {
            _renderTexture = new RenderTexture((int)_cameraRenderSize.x, (int)_cameraRenderSize.y, 24);
        }
        
        private void Update()
        {
            if (_camera != null)
            {
                Scene previousScene = _camera.scene;
                CameraClearFlags previousClearFlags = _camera.clearFlags;
                RenderTexture previousTexture = _camera.targetTexture;
                
                //tells the camera to render the prefab scene if it's in one
                if (PrefabUtility.IsPartOfAnyPrefab(_camera))
                {
                    PrefabStage stage = PrefabStageUtility.GetPrefabStage(_camera.gameObject);
                    if (stage != null)
                    {
                        _camera.scene = stage.scene;
                    }
                }

                //i use clear depth for my cameras so i want to override the clearflags
                _camera.clearFlags = _clearFlags;
                
                _camera.targetTexture = _renderTexture;
                _camera.Render();
                
                //reset camera values to what they were
                _camera.targetTexture = previousTexture;
                _camera.scene = previousScene;
                _camera.clearFlags = previousClearFlags;
            }
        }

        private void OnDestroy()
        {
            if(_renderTexture != null)
            {
                _renderTexture.DiscardContents();
                _renderTexture = null;
            }
        }

        private void OnGUI()
        {
            //inspector selector for the camera to draw
            _camera = EditorGUILayout.ObjectField(_camera, typeof(Camera), true) as Camera;

            EditorGUI.BeginChangeCheck();
            
            _cameraRenderSize = EditorGUILayout.Vector2Field("Camera Render Size", _cameraRenderSize);
            
            if (EditorGUI.EndChangeCheck())
            {
                //refresh the render texture to the new dimensions
                RefreshTexture();
            }

            if (_renderTexture != null)
            {
                //use the rect of the window but bump it down a little for controls
                Rect cameraTextureRect = new Rect(position);
                cameraTextureRect.x = 0;
                cameraTextureRect.y = CONTROL_AREA_HEIGHT;

                float windowWidth = cameraTextureRect.width;
                float windowHeight = cameraTextureRect.height - cameraTextureRect.y;
                
                float camRatio = _cameraRenderSize.x / _cameraRenderSize.y;
                float windowRatio = windowWidth / windowHeight;
                
                //fit the texture into the window at correct aspect
                if (windowRatio < camRatio)
                {
                    float ratio = windowWidth / _cameraRenderSize.x;
                    cameraTextureRect.height = _cameraRenderSize.y * ratio;
                }
                else
                {
                    float ratio = windowHeight / _cameraRenderSize.y;
                    cameraTextureRect.width = _cameraRenderSize.x * ratio;
                }

                EditorGUI.DrawTextureTransparent(cameraTextureRect, _renderTexture);
            }
        }
    }
}