I haven't remembered where I got that from but I have just found virtually identical info quoted here,
http://www.ukmotorhomes.net/motorhome-mot.shtml
Further down the page it seems large motor homes are possibly being seen as living vans if they carry bikes etc. as they are seen as "goods or burden", this may be very relevant for many members if not to this particular thread/discussion .
It is interesting to note that the DfT picked up on a slight error in the VOSA reply, just shows how complicated it all is.