I too believe that other vehicles past the truck after it was stopped and up until discovered. It is too long for there not to have been others going in to feed livestock, start restaurants, deliver papers, and a hundred different reason why you see unexpected movement on lonely roads. Surely there is more than one person who went in to "feed" and returned by the truck.
In todays world a person not recognizing the truck might just stop to "square him away" on his parking there. And to see it, know it, under the circumstances described, to not stop and at least see if there was not an ill driver inside.
There is so much mystery in why it was left there; no one wanted to have anything to do with it, and the one who almost wrecked, recognized it, and blithely tooled on off. It is almost as if David/his truck had become untouchable and everyone kept a long arms length. I saw, I ignored, I know nothing. That is not human nature folks, even in Iowa.