The biggest threat to the longevity of your RV cover is the wind. Movement due to billowing and sustained wind causes the cover to rub against hard surfaces and wear through the fabric. The idea is to get the cover on the RV as tightly as is possible. All covers come with bottom, front and back cinches to tighten, and the best covers have an additional cinch around the bottom of the body to prevent billowing.
But if your cover is bigger than it should be, it won't be possible for those cinches to gather enough of the excess fabric to get the tightness required. Conversely, if the cover is too small, it will be difficult to get it on the RV and you could tear the fabric just installing it. So it's important to purchase the correct size range.
As long as you are purchasing the correct cover for your type of RV, the width and height measurements are not relevant (except for some extra tall fifth wheels.) The manufacturers know what widths and height ranges are normal for a particular RV type (Class A, Class B, Travel Trailer, etc.)
The key measurement you need to have is the length, which should include the measurement from the very front of the RV to the very back. DO include your rear bumper, tire carrier or ladder. DO NOT include the tongue on a travel trailer, only the body. If you have a trailer with a rounded nose, measure from the furthest point forward. For a motorhome, start at the front of the bumper.
Once you have the length measured, select the cover in the correct length range. If your RV is, for example, 23 feet, and your choices are 20'-23' or 23'1"-26', get the 20'-23' version. If your length is 23'1", get the larger one. The covers have extra fabric built-in, so they're actually a little larger at the largest length, so you don't need the next largest size up if you're on the line between ranges.
Also, there is always enough room to account for any air conditioners, satellite dishes, or other protrusions on the roof.