As far as the "in public" part goes, the car on the road is in public.
As far as the the 647 (f) PC you have to not only be intoxicated but also "in a condition that he or she is unable to exercise care for his or her own safety or the safety of others" or such that it "interferes with or obstructs or prevents the free use of any street, sidewalk, or other public way."
I don't see how simply sleeping it off in the right seat qualifies. I'd talk to a lawyer.
I'm not a lawyer, but I did stay in a Holiday Inn Express last night.