When my cover was new, (ADCO) I would lay it as flat as I could on the driveway and spray it with a hose and using a shop broom sweep the dirty spots.. Didn't use any kind of soap or cleaner... Even thou it was a nice sunny day, it took awhile for it to dry, since it was mostly laying on the ground and all..
Well, that was the last time I washed it! :)
Still got it after 10 years now and it's a mess, but the inside where it touches the trailer is still clean and no rips or anything.. It's just a mess on the outside, but most of it is on the roof (actual MOSS growing on it!)... After it all dries it does flake off some... ;)

The only machines I could think of would have to be those industrial Hotel ones that are like 6' in diameter and all....
At least if your cover is dirty, it's doing it job and the reason you use it on the RV! :)
Good luck!
Mitch