I have been working on a Camera Controller for Unity that can be used for large area scenes. The code is not entirely mine but I believe it is worth sharing since I couldn't find something similar that easily.

CameraController.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;

/*
* In order for the code below to work without error you have to:
* 1. Add the script to the Main Camera
* 2. In Inspector change the Main Camera tag to MainCamera
* 3. In Hierarchy create UI element EventSystem
*/
public class CameraController : MonoBehaviour
{
    //Pan
    private float sensitivityX;
    private float sensitivityY;
    public Vector3 MousePosV3;

    //Zoom
    public int distanceMin = -100;
    public int distanceMax = 500;
    public float zoomSpeed = 50f;
    private Vector3 moveDirection = Vector3.zero;

    // Orbit
    public float minY = -45.0f;
    public float maxY = 45.0f;

    public float OrbitsensX = 100.0f;
    public float OrbitsensY = 100.0f;

    float rotationY = 0.0f;
    float rotationX = 0.0f;

    // Check Mouse on UI Layer
    static public bool checkMouseOnUI;

    private Vector3 currentMainCameraPos;
    private Quaternion currentMainCameraRot;


    public bool isOnable = true;
    readonly bool isPan = true;
    readonly bool isOrbit = true;
    readonly bool isZoom = true;

    public bool rockTarget = false;

    #region 
    public virtual void IsEnable(bool b)
    {
        isOnable = b;
    }
    #endregion

    private void Start()
    {
        rotationY = -transform.localEulerAngles.x;
        rotationX = transform.localEulerAngles.y;

        currentMainCameraPos = transform.position;
        currentMainCameraRot = Quaternion.Euler(transform.eulerAngles);
    }

    private void OnEnable()
    {
        RaycastHit hits;
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

        if (Physics.Raycast(ray, out hits, 10000))
        {
            MousePosV3 = new Vector3(hits.point.x, hits.point.y, hits.point.z);
        }
    }

    private void Update()
    {
        if (!isOnable)
        {
            if (Input.GetMouseButton(0))
            {
                isOnable = true;
            }
            return;
        }

        if (isOnable)
        {
            if (checkMouseOnUI == false)
            {
                if (EventSystem.current.IsPointerOverGameObject())
                {
                    return;
                }

                // Pan Start
                if (isPan)
                {
                    if (Input.GetMouseButtonDown(0))
                    {
                        RaycastHit hits;
                        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

                        if (!rockTarget)
                        {
                            if (Physics.Raycast(ray, out hits, 10000))
                            {
                                MousePosV3 = new Vector3(hits.point.x, hits.point.y, hits.point.z);
                            }
                        }

                        float distance = Vector3.Distance(MousePosV3, transform.position);
                        sensitivityX = distance * 0.02f;
                        sensitivityY = distance * 0.02f;
                    }

                    float deltaX = Input.GetAxis("Mouse X") * sensitivityX;
                    float deltaY = Input.GetAxis("Mouse Y") * sensitivityY;

                    if (Input.GetMouseButton(0))
                    {
                        transform.position += deltaX * transform.right * -1;
                        transform.position += deltaY * transform.up * -1;
                    }
                }
                // Pan end

                //Zoom Start
                if (isZoom)
                {
                    moveDirection = new Vector3(0, 0, Input.GetAxis("Mouse ScrollWheel") * zoomSpeed);

                    if (!Input.GetKey(KeyCode.LeftShift))
                    {
                        if (Input.GetAxis("Mouse ScrollWheel") > 0)
                        {
                            if (Mathf.Floor(transform.position.y + moveDirection.y) > distanceMin)
                            {
                                transform.Translate(moveDirection, Space.Self);
                            }
                        }
                        else if (Input.GetAxis("Mouse ScrollWheel") < 0)
                        {
                            // We shouldn't be allowed to zoom out more than distanceMax
                            if (Mathf.Floor(transform.position.y + moveDirection.y) < distanceMax)
                            {
                                transform.Translate(moveDirection, Space.Self);
                            }
                        }
                    }
                }
                // Zoom End

                // Orbit Start
                if (isOrbit)
                {
                    if (Input.GetMouseButton(2))
                    {
                        transform.RotateAround(MousePosV3, Vector3.up, Input.GetAxis("Mouse X") * 2);
                        transform.RotateAround(MousePosV3, transform.right * -1, Input.GetAxis("Mouse Y") * 2);
                    }
                }
                //Orbit End
            }
        }
    }

}