If you are satisfied with what Freeview offers, you are only intending to use it in the UK, and are prepared to accept that you may not be able to get TV on some sites, then an ordinary TV aerial will do the job. Many people on here will suggest the Status 530 which does the job but (a) needs to be mounted on the 'van and (b) needs space underneath it for the extending shaft.
If your van does not have one already fitted, then an external aerial will be needed. There are plenty of kits around, most of which are designed to be attached to the A frame (the bit behind the towball) in some means or other. I used to have a set of push together tubes, pushed the bottom into the ground and tied the pipe to one of the front handles - never had an problem.
If you want essentially guaranteed TV and/or intend to go abroad then get yourself a satellite setup. There are many good sites for advice, such as www.satcure.co.uk and www.satelliteforcaravans.co.uk. Get a minimum 60cm dish and you should be OK for most of Scotland/France/Germandy/Benelux/North Spain, but if you want to go to other parts of Spain or Italy you may need up to a 1m dish. If you are going to France buy your mechanical kit over there as it much cheaper in the DIY sheds. For a sat box either get a UK Freesat box or a secondhand Sky box with a Freesat-from-Sky viewing card (once off