I got into a disagreement with my boss for testing a van (class 7 £35) as a campervan (class 4 £25) when it had no windows in the back I phone VOSA (formally VI) and the definition they gave was "it had to have cooking and washing facilities, storage and seats all of which must be securely fixed, a table and some sleeping arrangement these may be removable/convertible but there is no requirements for windows " I also phoned the MOT helpline who said more or less the same thing so it was a big two fingers up to the boss