Fixing Issues with custom RTS Camera Controll Script

I don't really see why the code would not work, but I can show you how I did something similar (but for a TBS game).

 #pragma strict
 
 class CameraControls extends MonoBehaviour{
     
     public var speed: float = 5.0f;
     public var terrain: GameObject;
     public var xRotator: GameObject;
     private var terrainHeight:float;
     private var t:float = 0.0f;
     private var mouseClickPosition:Vector2;
     
     private function moveRight():void{
         if(Input.mousePosition.x > Screen.width * 0.95f || Input.GetKey(KeyCode.RightArrow)){
             this.transform.Translate(Vector3.right * this.speed * Time.deltaTime);
             this.getTerrainHeight();
         }
     }
     
     private function moveLeft():void{
         if(Input.mousePosition.x < Screen.width * 0.05f || Input.GetKey(KeyCode.LeftArrow)){
             this.transform.Translate(-Vector3.right * this.speed * Time.deltaTime);
             this.getTerrainHeight();
         }
     }
     
     private function moveForward():void{
         if(Input.mousePosition.y < Screen.height * 0.05f || Input.GetKey(KeyCode.DownArrow)){
             this.transform.Translate(-Vector3.forward * this.speed * Time.deltaTime);
             this.getTerrainHeight();
         }
     }
     
     private function moveBackward():void{
         if(Input.mousePosition.y > Screen.height * 0.95f || Input.GetKey(KeyCode.UpArrow)){
             this.transform.Translate(Vector3.forward * this.speed * Time.deltaTime);
             this.getTerrainHeight();
         }
     }
     
     private function getTerrainHeight(){
         var hit: RaycastHit;
         this.terrain.layer = 0;
         // Get terrain y value at current position.
         if(Physics.Raycast(Vector3(this.transform.position.x, 300, this.transform.position.z), -Vector3.up, hit, 500)){
             this.terrainHeight = hit.point.y;
         }
         this.terrain.layer = 2;
     }
     
     private function adjustToTerrainHeight(){
         if(this.transform.position.y != this.terrainHeight){
             this.transform.position.y = Mathf.Lerp(this.transform.position.y, this.terrainHeight, this.t);
             //this.transform.position.y = this.terrainHeight;
             Debug.Log("y " + this.terrainHeight);
             t += Time.deltaTime;
         }
         else{
             this.t = 0;
         }
     }
     
     private function rotateCamera():void{
         if(Input.GetMouseButtonDown(1)){
                 this.mouseClickPosition = Input.mousePosition;
         }
         if(Input.GetMouseButton(1)){
             var delta = this.mouseClickPosition - Input.mousePosition;
             this.mouseClickPosition = Input.mousePosition;
             this.transform.Rotate(Vector3.up * -this.speed * delta.x * Time.deltaTime);
             this.xRotator.transform.Rotate(Vector3.right * this.speed * delta.y * Time.deltaTime);
             this.xRotator.transform.rotation.eulerAngles.x = ClampAngle(this.xRotator.transform.rotation.eulerAngles.x, 0.0f, 40.0f);
         }
     }
     
     private function ClampAngle(angle: float, min: float, max: float): float {
         if (angle<90 || angle>270){       // if angle in the critic region...
             if (angle>180) angle -= 360;  // convert all angles to -180..+180
             if (max>180) max -= 360;
             if (min>180) min -= 360;
         }   
         angle = Mathf.Clamp(angle, min, max);
         if (angle<0) angle += 360;  // if angle negative, convert to 0..360
         return angle;
     }
     
     public function Update(){
         if(!Input.GetMouseButton(1)){
             this.moveRight();
             this.moveLeft();
             this.moveForward();
             this.moveBackward();
         }
         if(Input.GetKeyDown(KeyCode.Escape)){
             Application.Quit();
         }
     }
     
     public function LateUpdate(){
         this.adjustToTerrainHeight();
         this.rotateCamera();
     }
 }

The camera setup is like this (as tree):

You won't probably need the terrain. I need it to switch its layers dynamically because I don't want my grid to hit it with raycasts to determine the moving radius with built-in obstacle detection.

The script is attached to the yRotator.

For the zooming I would do it in a separate script, use Translate(Vector3.forward speed Time.deltaTime) and attach it to the main camera (in the setup i showed you here) or add another public variable for the main camera. Speed should of course be negative or positive depending on the scroll direction.

Maybe you can get some inspiration from it.

C# version (no guarantee for correctness)

 using UnityEngine;
 using System.Collections;
  
 public class CameraControls : MonoBehaviour{
  
     public float speed = 5.0f;
     public GameObject terrain;
     public GameObject xRotator;
     private float terrainHeight;
     private float t = 0.0f;
     private Vector2 mouseClickPosition;
  
     private void moveRight(){
        if(Input.mousePosition.x > Screen.width * 0.95f || Input.GetKey(KeyCode.RightArrow)){
          this.transform.Translate(Vector3.right * this.speed * Time.deltaTime);
          this.getTerrainHeight();
        }
     }
  
     private void moveLeft(){
        if(Input.mousePosition.x < Screen.width * 0.05f || Input.GetKey(KeyCode.LeftArrow)){
          this.transform.Translate(-Vector3.right * this.speed * Time.deltaTime);
          this.getTerrainHeight();
        }
     }
  
     private void moveForward(){
        if(Input.mousePosition.y < Screen.height * 0.05f || Input.GetKey(KeyCode.DownArrow)){
          this.transform.Translate(-Vector3.forward * this.speed * Time.deltaTime);
          this.getTerrainHeight();
        }
     }
  
     private void moveBackward(){
        if(Input.mousePosition.y > Screen.height * 0.95f || Input.GetKey(KeyCode.UpArrow)){
          this.transform.Translate(Vector3.forward * this.speed * Time.deltaTime);
          this.getTerrainHeight();
        }
     }
  
     private void getTerrainHeight(){
        RaycastHit hit;
        this.terrain.layer = 0;
        // Get terrain y value at current position.
        if(Physics.Raycast(Vector3(this.transform.position.x, 300, this.transform.position.z), -Vector3.up, hit, 500)){
          this.terrainHeight = hit.point.y;
        }
        this.terrain.layer = 2;
     }
  
     private void adjustToTerrainHeight(){
        if(this.transform.position.y != this.terrainHeight){
          this.transform.position.y = Mathf.Lerp(this.transform.position.y, this.terrainHeight, this.t);
          //this.transform.position.y = this.terrainHeight;
          Debug.Log("y " + this.terrainHeight);
          t += Time.deltaTime;
        }
        else{
          this.t = 0;
        }
     }
  
     private void rotateCamera():void{
        if(Input.GetMouseButtonDown(1)){
           this.mouseClickPosition = Input.mousePosition;
        }
        if(Input.GetMouseButton(1)){
          Vector2 delta = this.mouseClickPosition - Input.mousePosition;
          this.mouseClickPosition = Input.mousePosition;
          this.transform.Rotate(Vector3.up * -this.speed * delta.x * Time.deltaTime);
          this.xRotator.transform.Rotate(Vector3.right * this.speed * delta.y * Time.deltaTime);
          this.xRotator.transform.rotation.eulerAngles.x = ClampAngle(this.xRotator.transform.rotation.eulerAngles.x, 0.0f, 40.0f);
        }
     }
  
     private float ClampAngle(float angle, float min, float max) {
         if (angle<90 || angle>270){       // if angle in the critic region...
             if (angle>180) angle -= 360;  // convert all angles to -180..+180
             if (max>180) max -= 360;
             if (min>180) min -= 360;
         }   
         angle = Mathf.Clamp(angle, min, max);
         if (angle<0) angle += 360;  // if angle negative, convert to 0..360
         return angle;
     }
  
     public void Update(){
        if(!Input.GetMouseButton(1)){
          this.moveRight();
          this.moveLeft();
          this.moveForward();
          this.moveBackward();
        }
        if(Input.GetKeyDown(KeyCode.Escape)){
          Application.Quit();
        }
     }
  
     public void LateUpdate(){
        this.adjustToTerrainHeight();
        this.rotateCamera();
     }
 }