When you chose a cover, you will want to ensure it is compatible with your specific RV, but you also want to sure that it fits the specific size and measurements of your RV. It should fit snugly to prevent moisture, dust, or wind getting underneath.
The complication is that when you measure height, length, and width, you need to include all the other exterior elements that should add inches to your measurements. Your RV likely includes storage racks, ladders, bumpers, and mirrors.