Is there such a thing as a mobile caravan cleaner (for outside of van)?

Yes....
One such company is Carashine based in the North if I remember correctly.
Telephone: 0795 5645405
The website appears off-line for some reason.
http://www.carashinenational.co.uk/index.htm

It seems the URl has changed.
http://carashinenational.co.uk/index.html

Thanks Allen. We used carashine. Did a fantastic job, caravan looks like new. Will be having them once a year from now on.