I have not done this yet. However, while looking for a care label, I found that the cover is made by coverking. On coverking's site, there is a FAQ stating that you should hand wash with a mild soap by gently swishing around the cover in a large basin. Hang dry.
Looks like the bathtub is my only option.
If for some reason you just want to wash the outside of the cover, they mention using a mild auto soap while the cover is on the vehicle.