I'm no legal expert but I think when they're parked up in an appropriate place (private land, layby, that kind of thing) they're considered a temporary residence? Who knows with our convoluted english common law!
It's very much still a vehicle, but there's a statutory defence to Drunk in Charge if you can demonstrate that there was no likelihood of driving while you were over the limit. Having a bed in the back and being parked in a suitable place to spend the night should be more than enough to demonstrate that unless you're seen to be climbing into the drivers seat.
In the UK, this may be your own private domicile, but you can still be harrassed.
