I have found in driving a 18 wheeler the past 15 years, and a tour bus too. I use the right side mirror and wait until I see both headlights of the truck I'm passing. If I see the full front of that truck I know my truck and my 53 foot trailer will clear and I can start to move over. Besides, most drivers will give you the flash of the headlights that you are clear. To test this out, use a parking lot with a car parked. Pull up until you see the full front of that vehicle. Stop and get out and look at the back of your RV. How much clearance do you have? If needed get in your RV and pull forward 10 feet and look at the picture in that right side mirror. Once you have the clearance you are looking for, remember what it looks like in that mirror. Also the use of Shadows must be used when you have them. In backing up or passing a vehicle, use the shadows of the items around your RV to tell you of dangers. It all comes with practice and experience of moving your 40 foot or plus RV through the parking lot or down the road.