I live in wales and have cleaned caravans here!
You can charge a decent wage and even get some repeat custom.
Caravans are a different ball game though, some can be very hard work and have staining etc that you cant shift!
One of my usp's was that the wfp is the "safest" method to clean a caravan. If you look into it you are not supposed to clean a caravan with a jet wash as they cause damage and water ingress , apparently even the caravan manufacturers don't recommend it.
All they have on offer where I am (north wales) is jet wash and I never heard a single bit of good feedback from it.