Whilst the concept is reasonable, the figures just do not add up.
Also the use of steam on a caravan is not particularly appealing due to the different materials used in manufacture.
To start with I am presuming you intend to be mobile? If you are then some places will not have power so a generator is needed.
If so then you have to put into your charging costs the price of the machinery, cost of vehicle, road tax, insurance, mot, wear and tear and servicing, fuel costs, business insurance, personal indemnity, product indemnity, and of course your wages, and those who work with you, plus the tax burden.
Take into consideration that many owners only clean their vans once or twice a year and you will see that the market is very limited, and with the current economic situation one of the first things to be shelved would be paying to do something the owner can do themselves.
Having seen a caravan valet at work I can assure you that it takes a lot longer than the equivalent of one hour to clean a van completely, and the state of some they can take all day.
I think you need to really research your idea and work out proper costings.
With all due respect to the contributers to the forum, you need to research your area, or the area you intend to offer your service to.
Look around, how many vans are there? are there dealers who may be interested? are there any storage sites?
If all the figures work out, then good luck, but I am sure you will have to charge a lot more than