To ensure your room darkening pull down shades fit perfectly on your 2018 Grand Design 5th Wheel, measure the length from just below the top metal mounting rail where the shade attaches to the wall, down to the bottom metal rail that you pull the shade down with. This measurement will capture the full coverage area of the shade material including the metal rails, ensuring a proper fit. For width, measure the total width you want the shade to cover, adding about one inch on each side if you plan an outside mount for better light blockage. Use a steel tape measure for accuracy and double-check all measurements before ordering. This method guarantees the shades will fully cover your windows and function correctly.