Depends on how you intend to use the van.
It's easiest, initially, to try to insure it as a van or a dual-purpose vehicle, as these are catered for by the generic comparison websites and you can [relatively] easily obtain quotations from a wide range of insurers. If you can't get a decent price there, you could then try the specialist campervan brokers, but that takes a lot more effort as you have to ring each individual broker for a price.
As above, wherever you try, be sure to declare all the mods and be honest about your intended use, otherwise you risk having your policy voided and being in a world of pain when you next try to get insurance.