@Becky489 A few points to help you
The calendar in Airbnb needs to be kept up to date by you. If the caravan is marketed elsewhere you might be able to link the calendar to the other booking site. Perhaps switch off Instant Book until you get this under control. You will be fined up to $100 if you have to cancel a duplicate booking.
When a booking is taken Airbnb collect the money and release it to you after the guest's stay has started.
There is no charge for listing on Airbnb. You will be charged 3.6% of the rental money and the guest will pay an additional 17% of the nightly/cleaning fees you charge unless you are on the host only fee model where you will be charged 18% ( I think) of the rental charges.