Ok, so I think you've partially answered your own question - He isn't at the "you don't have to watch him" stage yet, so keep him in sight.
You also don't mention if he gives you any signal (when he is in sight and needs to pee) - if he doesn't, consider bell-training him, so you get an audible signal from him "hey, I gotta go!"
As for the poo accident, yes, they can get that distracted! I've had my pups be "ready to burst" on their back ends but then a dog walks by and they completely forget what they are there for. You mention bringing him to a potty spot - that is very good, but you can't let him play there. You need him to know that the spot is for potty only, play comes later (or during a separate trip). And even though I'm sure you were freezing your hiney off, you need to wait him out if you *know* he needs to go and hasn't gone. By putting him in the truck, you were setting him up for an accident since he didn't have a place to go and still had a "full tank."