A cover can't stop moss or mold from growing. The spores are already there.
I never use a cover in the summer. Too much work to put on and off each time I go camping.
However, to answer your question, what you need to know to order a cover is the dimensions of your RV.
To prevent the cover from ripping apart in the wind you will need a properly fitting cover, properly strapped down and secured. You also need to put down your radio antenna and cushion any sharp corners such as gutter drains.
Remember, good fit, properly strapped.
I bought a cover from Amazon 4 years ago and it is still in service. It's similar to this ADCO
You can spend a whole lot more on a custom-sized cover.