Wording from the letter in @Oldrat 's post #503
Re: "Van with side windows' speed limit
Thank you for your online enquiry of 26 July to the Department for Transport (DfT) regarding the speed limit of your recently purchased campervan. It has been passed to me in the Driver and Vehicle Licensing Agency (DVLA) to respond.
The national speed limits for different categories of road vehicle depends on how the vehicle is configured for use on the road. That generally means whether the vehicle is designed primarily for the carriage of passengers or goods.
The 'body type' field on the DVLA vehicle record should not be used to determine a vehicle's speed limit as this is used purely to describe a vehicles external appearance in traffic, to assist the police and other enforcement agencies. A vehicle with ‘van with side windows' body type does not necessarily mean it is restricted to speed limits of a goods vehicle.
Provided you are satisfied the vehicle meets the internal reguirements to be recognised as a motor caravan and any alterations made to the vehicle do not compromise its safety, then & should be treated as having a speed limit of a motorhome or motor caravan in the guidance found here - www.gov.uk/speed-limits
We do not distinguish between campervans or motorhomes for the purposes of changing the body type of a vehicle to motor caravan on the registration document. All campervans, motor caravans and motorhomes fall under the category of "motor caravan'. There are no plans to change this.
I hope the helps to clarify the position.