Yep there are no real cleaning advantages of getting a fitted system. A diy will clean the windows just as well. Having a tank set up in a shed or garage though is a very good idea, ro's in vans aren't really ideal, especially in winter months.
Fitted systems are good though in that you don't have to fit it yourself, you let some one else worry about it, but it won't work any better than a good diy system. I'd go diy and spend the extra money on decent poles,that's where the real advantage will be.
Simon.