I did a VW T5 conversion with elevating roof with rock n roll bed and full kitchen . Just the build was around 8k . I had DVLA change the logbook from commercial to
motorcaravan .This was about 10 years ago ,they have changed the criteria now by not recognising an elevating roof. However if you have a high top commercial van and want to convert it to a camper add side windows, cooking facilities, bed, seats & decals on van to make it look like a camper
Theres a good chance it will be passed as a motor caravan . If not it’ll be passed as a van with windows you’ll still be able to