It's fairly easy if you follow the guidelines. Have a look on the DVLA website, there's a section on how to do it which shows what is acceptable and what isn't acceptable for the vehicle to be reclassified.
The difficulty is all in the conversion. To be a campervan/motorhome needs certain items, cooker etc permanently installed. As pointed out search online for actual spec required.
For example you can get VW T4 & T5 often lhd imports that have lifting roofs & folding beds but without the rest of the permanently fitted extra equipment cooker/sink etc they cannot be legally registered as campervans.
Here's a cut and paste from the DVLA website (can't do the link thing) ...
a door that provides access to the living accommodation.
a bed, which has a minimum length of 1800mm or 6 feet - this can be converted from seats used for other purposes during the day but must be permanently fixed within the body of the vehicle.
a water storage tank or container on, or in, the vehicle
a seating and dining area, permanently attached to the vehicle - the table may be detachable but must have some permanent means of attachment to the vehicle. It is not good enough to have a loose table
a permanently fixed means of storage, a cupboard, locker or wardrobe
a permanently fixed cooking facility within the vehicle, powered by gas or electricity
at least one window on the side of the accommodation...
Last 10 years I've had two vans reclassified, pretty straight forward really.
Take pictures of the van and additional equipment and submit them with a few invoices as proof of conversion along with your V5 document.
We bought a PVC which had been newly converted by Devon Conversions, they gave us a standard letter from themselves to the DVLA with photos, and even included the stamped, addressed envelope! I sent it off last week when the new V5 arrived