Christine, in most instances the caravan will be covered for 3rd party risks whilst attached to the car.
It is ESSENTIAL to inform your insurer that you will be towing, even though there will probably be no extra charge, and follow up any phone call with a letter!
Additional cover for the caravan is not mandatory, but extremely wise, unless you have limitless money should an accident happen when on site, siting the van, or when not attached to the car.
For example,,,,,you unhitch the van at your destination, and the brakes do not hold it where it is intended to be and the van runs down an incline into another van,,,,,,if you do not have the extra insurance, the cost of repairs to your van and the one yours hit is down to you.