To be clear, the Body Type shown on the V5 has zero relevance to insurance. Anyone with a campervan needs campervan insurance, otherwise they risk only receiving the market value for their base van in the event of a total loss.
Anyone, like yourself, who is currently mid-conversion, should get specialist conversion insurance that's specifically designed to support this process. Whilst it's possible to convert on standard van insurance, it can get very expensive very quickly as each conversion step has to be declared as a modification, each of which will probably be charged for as a mid-term adjustment (including admin fees). There's also the very real possibility that, midway through the policy, the insurer refuses to provide ongoing cover, due either to the number of mods or the nature of a combination of the mods.