I am part of the technical team behind www-dot-caravansitefinder-dot-co-dot-uk which has an online booking engine.
We have an API that is used by software providers such as www-dot- goughware-dot-com to provide online availability and booking from camping sites and caravan parks.
The web site is owned and administered by Haymarket (www-dot-haymarket-dot-com) so you would be dealing with them on the business side.
You could either use Goughware directly or use our API to provide your online booking services.
If you have not already got a solution, good luck!