Unless Revolution Campers have type approval, then insurers will view their conversions as a series of modifications and, as you've already discovered, most won't offer Vehicle Replacement GAP. There may be a niche insurer out there that offers what you're looking for, but I'm afraid I'm not aware of one.
An alternative approach would be to purchase an Agreed Value policy so that you're covered for the full replacement value in the event your van is written-off. You'll usually need to provide a purchase invoice, pictures of the van and a detailed description of the conversion in order to justify the value you wish to agree and I'd advise you review & revise that value annually. It can be a tedious process, but it may be a more realistic way to achieve what you want.