Outdoor covers by their very nature have to meet multiple criteria. To that end various products have attributes that affect durability in cold or wet or sun-baked conditions. As such different applications result in different materials.
The best advice for proper cover care is as follows
DO NOT store covers when wet. While it may be convenient to fold up covers and place them in a storage box after a rain, it is very important to remember to get them out to air dry as soon as possible. Regardless of "mildew" resistance, a wet cover will gather mildew if stored wet.
DO NOT use detergents to clean covers. Covers are normally coated to improve water and mildew resistance. Detergents break down that coating and will reduce the performance of the cover. Detergent residue mixed with the elements such as sun may cause fading of colour. It is best to use warm water and a soft brush to clean. You may try some mild soap in low mixture but if you do, rinse off well. REMEMBER to air dry the cover after.
DO NOT machine wash or dry. (see detergents above) While doing so is not critically bad, damage may still occur. Life of product will certainly be reduced. The product is not designed to withstand heavy duty mechanical washing and drying. If you prefer to machine wash, use smaller amount of soap on a gentle cycle.
DO NOT blast your cover with a pressure washer. It stands to reason that any fabric blasted with a pressure washer will be damaged by the intense spray.
An outdoor cover's function is to protect what is under it. Take good care of your cover and it will take care of your "stuff".
Covers are used to protect things from dirt and grime. So they are going to get dirt and grime on them. That's OK, better a dirty cover than "dirty stuff".