You don’t need motorhome insurance for a caravan that’s being towed.
If you’re towing a caravan, you only legally need standard insurance to cover the towing vehicle but that also means that if you are involved in an accident, only the car you are driving would be covered and any damage to the caravan would be your responsibility. It’s not a legal requirement to have specialist caravan insurance, but you can choose to take it out if you want your caravan to be covered.
A motorhome is different and you do legally need to have motorhome insurance to drive one.