Sunbrella is great stuff, it is costly but will last for many years.
If you have one made, make sure they put some breathable vents in it so you avoid mold and have them put a door zipper in so you can get into your camper without taking the cover off.
You also need to protect/pad any hard edges like the gutter spouts or they will eventually tear through the cover...
I have an ADCO breathable.. It fits well, has a good tie down system and they are a good value. However, from most accounts, they won't last more then 4 or 5 years.
2014 Roo 21DK