Welcome Jason! Great question. Measure the length of the RV from the two longest points on the vehicle, not including the hitch. Then measure the height of the RV from the top roofline down to the outside frame. Do not stop at the top of the wheel well or measure all the way to the ground. And do not include any rooftop accessories like AC units or vents in your measurement.
Here’s a guide eon exactly how to do it with visuals: https://camperfaqs.com/how-to-measure-for-rv-cover
Let us know if you have any more questions!