I sat as a Magistrate for many years so I have seen this type of offense many times. The reality is, if a police officer suspects your intent was to drive or suspects you have previously been driving you can be charged with drunk in charge. The fact you may be on a private car park makes no difference. It may not surprise you to learn that may drink drivers are caught on the pub car parks as the police will wait for them if they receive a tip off.
I heard all sorts of spurious defenses in my time such as I was warming the car, I was sleeping in my vehicle. I have also seen drivers prosecuted for sleeping in the back of their van which has a separate cab. I personally agree with
@andyinluton if you have a prebooked stay at pub stoppover that would be little different to a campsite so the Police would be very harsh to charge you there.
Personally if I was staying in the van anywhere other than a campsite or my own private land I'd not drink. Losing your license for a few drinks is simply not worth the risk.