overnight the temperature can drop and if he wasn't prepared he got lost in the woods
Agree 100%. To me, objectively, taking the truck is impulsive. Subjectively, I'd say he was fully rattled rather than he found it an opportune time to steal something. Coupled with it appearing he had no possessions other than what he was wearing and was in his pockets, his actions were born of chaotic, nonordered thoughts. He just fled.
Added to that he was allegedly prescribed thorazine. If he was "taking it, but not taking it" around that time (i.e. inconsistently, skipping or full on withdrawal) there is no way to know his mental state, but likely beyond just 'despondent'. Especially at age 18 where the effects can be different, potentially a full break. Sadly, he was at least in psychiatric distress.
I'd guess he either went right into the woods from the truck, or walked the road to get some distance between it and himself and then veered into the woods. Otherwise he could've taken it farther, though I don't know how long an APB would take in those parts in 1977 or if that crossed his mind. He may have hitched a ride, but it sounds like he was avoiding people. Also, it's more likely a driver would drop him somewhere and potentially remember him once the story broke (or not, and he disappeared later to never been again) than be a random killer. Statistically, anyway.
On May 12th, he could've quite a distance in daylight. The danger of darkness slowing/stopping him and hyperthermia descending may have occurred to him too late to get back to help or potentially not occurred to him at all.
Is there any chance he had a gun on him, or stopped to get one? And do we know if he could swim?