actually, funnily enough in the UK you can be arrested for being "drunk in charge of a motor vehicle" even if you aren't in the driver's seat. That can apparently include sleeping one off in the back seat of your car.
Amusingly, here in the UK, we also have the crime of "drunk in charge of a carriage" which has been used to prosecute people for being drunk with all manner of things from bicycles to horses.
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.