It was registered to the converter when delivered to them on 12/04/18 although we paid the full cost of the base vehicle. Even the recorded date of transfer to us of registered keeper 06/07/18 is actually two days after we took delivery, on 04/07/18.
As I said, we were not pleased! But we plan to be in for the long term, love the van and want to move on from our displeasure.
This is because of type approval and yes the converter has mis-sold the product if he stated it was a 'new' van and hence you would expect to be the first registered keeper.
Legally, to have the registration issued as 'New at first registration' in the customer's name with body type 'Motor caravan' would have required the converter to purchase an M1 van derivative with an incomplete Certificate of Conformity. The converter would then have to supply type approval paperwork at registration (either EWVTA, NSSTA or IVA) for a 'completed' certificate of conformity.
Change of body type to 'Motor Caravan' with a pre-registered vehicle is a submission of photographs and documentary evidence to DVLA which is a much simpler exercise.
That your van was not first registered as a motor caravan in your name is a workaround/cop out out by the converter to avoid having to go through type approval - if he definitely did not inform you of this process at point of sale you have been misled. You cannot sell 'New' vehicles without type approval since 2011.
Teesix is spot on in that you have been sold a second hand vehicle; regardless of the mileage or whether the only ever bum in the seat before yours was a German putting it on the transport. There are other implications to buying a 2nd hand vehicle - for example a lot of insurance companies will cover full replacement value of a vehicle under 1 year old IF you are the first registered keeper, some manufacturers warranties don't transfer (although this isn't the case with VW) etc.
This doesn't mean that it is a poor conversion or an unsafe van - just that it was not a new motor caravan when sold to you. If you were indeed mis sold the van, you have a couple of choices: demand your money back or seek through small claims any 'losses' you have incurred.
In my experience of selling campers, there is much less emphasis placed on number of previous keepers as there is with cars. Nowadays with the number of pre-registered cars/vans hitting the market, I think it is of negligible value being '1 owner'. If I had to put a figure on the decrease in value due to having 2 keepers rather than 1, I'd say in the order of a thousand pounds max.