You'll most likely have to hire the van - one thing to think about well in advance, and then hire the pitch on the site, another thing to do in advance?
There are places which hire out camper vans but do your homework and check out the reviews. There are good places, like a few of the companies who build the vans (Jerba) and there are bad sites, like a place I forget, on outskirts of Edinburgh who advertise a nice van but when you arrive, give you a false story about the customer returning the van with ripped seats and try to palm you off with their 'spare' van which is very old and disgusting. I checked the company out on Companies House - a register of all UK listed companies, and the owners went from similar named hire place to similar hire place. So as I say, ask questions, do your homework, book on a credit card, so you have protection against attempted fraud.
For Campsites, you can turn up at sites and hope they have space, and if they don't - depending on location and time of year, they will suggest an alternative, who they know locally, though the dates you give may be a bad time of year as some sites close Oct to March.
I'm sure there are other van hire places which others can recommend but Jerba (in Scotland) is a great place, though their hire vans do book up early. Google Jerba Campervans.