DVLA is required to record vehicle details for road safety and law enforcement. The DVLA body type is used for establishing vehicle appearance and identification and does not affect the insurance category of the vehicle or have any effect on speed limits or legislative requirements. To change the body type of a vehicle, DVLA must be satisfied that the vehicle meets the policy for body types. This means that the body type information held on the vehicle record must describe what a vehicle looks like in traffic.

DVLA will only consider changing the body type to ‘motor caravan’ (motor home) if the vehicle can meet the internal and external features.

While DVLA will consider all applications for a change to body type, there will be certain vehicles with an original frame or outer structure that is so distinctive in appearance that the body type description cannot be changed from the initial manufactured design, for example, a bus. If you’ve previously applied to convert your vehicle to a motor home or motor caravan and your application was rejected

Even if the vehicle’s body type is not changed to ‘motor caravan’, the vehicle can still be used for this purpose as long as the keeper is satisfied the converted vehicle meets the required internal features for motor caravans.

It is the responsibility of the vehicle keeper to make sure that any alterations made to the vehicle do not compromise its safety. If you sell the vehicle at a later date, it is your responsibility as the registered keeper to demonstrate the conversion work has been safely completed

https://www.gov.uk/government/publications/converting-a-vehicle-into-a-motor-caravan