Technically, I believe we are using only one "habitation unit" -(there's just the two of us, and we use only the caravan facilities on site, but use the motor van as a "day van" when out and about. - we found the Topaz a little TOO little for travelling AND sleeping in).
Most CC and C&cc site wardens accept this with no problem; We don't cook, wash or sleep in the VW on site, just as those holidaymakers with pup tents for the children don't expect to pay for two pitches when they all fit on one pitch! So the VW is just a rather over-the -top towcar when parked next to the caravan.
We did have a problem with one CC warden, who insisted we park in the visitors' area (but that was right next to our caravan, anyway!) And on another CC site, we had to take the gas bottles out of the VW and store them UNDER the CARAVAN for Health & Safety reasons!!!!. The official CC HQ response is that we have 2 "habitation units", which cannot go on one pitch, so we should remove the gas (from a secure, vented locker!), or park other than on the caravan pitch. I'm pleased to say that 99% of wardens don't know, or ignore, this advice/rule, and the official C&CC reply to my query agrees exactly with what I believe. So they must be right!
Seriously, if you do go down the Motor van + caravan road, it's probably worth phoning any site before travelling, to check what rules they want to go by. Most of those I have asked have been amazed that I should even have raised the question, though.