As my 'van is a very old and unique classic van, I had this problem when I first tried to insure it. The problem was that whilst the manufacturer was listed, mine is a one-off model, created for its original owner, and not in any makers catalogue. Therefore no-one could give a market value for it.
I finally went to a specialist company called Caravan Rescue (0845 345 4525), who do an agreed value policy. Its not particularly cheap, but proved its worth when I bumped the van a year or two ago, causing £2,500 worth of damage. As the van is insured for £8,000 the claim was paid without any problems.
Just as an aside to that, when I took the van to Discover for an estimate, they dismissed it as "an old van" and that the insurers would laugh at my claim. When I told him how much it was covered for, he rapidly decided that perhaps they would like to quote for job.Guess who
DIDN'T repair it?