I've just insured (comprehensive plus legal, with 90 days EU cover) my 2016 T6 Start line newly converted camper (plus alloys) for £250.00, it would have £20 cheaper, if I'd known it had a factory alarm beforehand.
I used a company called Staveley Motorhomes, which I found through the Go compare Motorhome website. As a camping and Caravan Club member, I get a discount. Their own insurance was almost £600 for similar cover. I went for 10,000 miles too, to be on the safe side!