As far as I’m aware, it is possible to register a camper van for disabled use. However, you can only have one vehicle registered for tax exemption and it must be registered in the name of the nominated driver. That means the camper would retain its disabled status, so long as it is the sole form of transport for your child. For more details, see: www.gov.uk/financial-help-disabled/vehicles-and-tr...t

Obviously, if you’re converting the van, you’ll need to return the V5C for amendment. However, I would recommend contacting the DVLA first to explain the fact that you need to retain the disabled status as there might be additional paperwork to fill in. See: www.gov.uk/contact-the-dvla