Personally I use the camc insurance,both major clubs do good insurance.As for levels,mainly I think there's new for old cover,where even if your van is a few years old it's replaced with the latest equivalent or nearest alternative or money equivalent to purchase the one you want.Or in the event of a total loss you get paid out what it's worth at time(same as most car insurance).
There are plenty of other caravan insurers but the major clubs seem to get the best thumbs ups when things go pear shaped,one thing I've noticed is there's lots of levels and prices(and small print)that's why I use the club,hope this helps,cheers.