I last camped under "canvas" at a festival about 10 years ago. Apart from being very wet, it was very noisy and a long walk to the loos! I do get the impressions that at Scottish festivals at least, there are fewer people in tents than there used to be and far more in camper vans or caravans, and as Jack says, a whole other village at some festivals, some of the occupants never actually coming into the festival centre. Ah, but who could blame them for getting a "van" with the Scottish weather, where tents have been known to end up in the Firth of Clyde, next seen in America!
While I have been tempted by the camper van idea, the actual hard economics of buying one, maintaining it, taxing it, fuelling it, paying campsite fees etc probably cost more than spending a few festivals in B & Bs or hotels.