Car/caravan insurance

My daughter is trying to lure her husband away from tenting to caravanning and I am helping by offering to lend my van to them for a trip away. However, they are struggling with their Insurance company (Budget) who claim that the van should be separately insured. I always thought that when the van is attached to the car then it's covered by the car insurance. I had a knock in France last year and this was borne out in that, the car ins. covered the claim. In addition, my caravan policy states that Caravanguard have no liability when the van is being towed or becomes detached from the car. Advice and opinions welcome.