From memory, the cases I've heard about of people being done for being drunk in charge when "sleeping it off" in the car, or even just getting a coat from the car (having phoned a taxi), normally relate to pub car parks etc - when the police could suspect that you may have had the intention to drive off to your comfy bed (until you saw them). I'm sure there must be other cases in other locations, but if we are talking about more remote areas and if you are parked next to your tent or have a bed roll laid out in the car etc (as already suggested) to suggest you have any intention of moving the vehicle would be another level of zelatory again!
But as rohan says - do you want to risk it?
Equally, and maybe I'm just a lightweight
, assuming I've just consumed a hearty carbohydrate laden meal, would I want to be drinking enough to put me over the limit, when I'm going to be up early for a strenuous walk?