Ferret,the relevant law is Sectio4 (2)of the road traffic act1988 which is Being in charge of a mechanically propelled vehicle on a road or other public place when unfit to drive through drink or drugs.
Section 4(3) however states that "a person shall be deemed not to have been in charge of a mechanically propelled vehicle if he proves that at the material time the circumstances were such that there was no likelihood of his driving it so long as he remained unfit to drive through drink or drugs"
Section 4(3) however does not "do away " with the original power to arrest for the section 4(2)offence.
CPS require, for anybody caught up in this process,to be interviewed whilst under arrest regarding thier intention to drive.So supporting evidence of when in a Motorhome overnight and no intention to drive could be ,front blinds on the windows ,any security on steering wheel fitted,any support legs down,the fact that your in bed under a duvet etc.
I,ve been overnighting for 13years now and never had one query from police possibly because its remote areas which rarely see police.
I appreciate what roi states but section 4(3) does apply.