[reworht]

It ain't cheap, and it's only available to Camping & Caravanning Club Members,, but RAC Arrival Cover is pretty comprehensive. They do cover Harry's hypothetical situation of the car breaking down away from the site - they will reunite you with your 'van before taking you wherever.

How Arrival breakdown cover benefits you if you have a caravan or trailer tent

Unique benefits for caravans, trailer tents and trailers include:

Caravan breakdown recovery on any UK campsite.
Cover for large units over 7.6 metres (25ft) in length or 2.3 metres (7 ft 6 in) in width^ .
If your caravan cannot be towed and requires recovery on a flatbed truck, RAC will recover it at no additional charge† .
‘Arrival Return’ so if you suffer a car or caravan breakdown on the way to a campsite and RAC can’t repair you at the roadside or at a local garage within a reasonable amount of time, they will take you, your car and caravan to the site and back home again†.
Follow on Recovery, so we can take your caravan to a second location of your choice (within a 75-mile radius)†.
A ‘Re-Unite’ service so if you are away from your campsite when your car breaks down, RAC will take you back to the site within a 75-mile radius to collect your caravan before getting you home†.
Optional European caravan breakdown cover with no supplements (for cars or caravans)
Personal breakdown cover, so you will be covered as a driver or passenger in any vehicle, whether you are towing or not.