Hi
The way I would approach this is work out a price for a basic clean and then a price for cleaning if the caravan was left really dirty. Add together and divide by two to get your average cleaning price per caravan. I should imagine most people will leave them in a state inbetween and there will be more to clean at 'basic' than at 'really dirty'.
The caravan site will be able to give figures of turnaround so you will then be able to allocate specific hours to the cleaners, the hard part then is getting the cleaners to understand that they have x amount of hours to clean x amount of caravans to a certain standard regardless of the mess they are in! The time should even out most days but for days you are running over there should be days you also run under.
This is not an easy quotation to get right and I hope the above is understandable!

Possibly speak with the management about the problems that may arise (they will already be fully aware) and suggest a review after a certain period of time for adjustments if needed.
Fox