You could leave your keys in the truck while it is running, but the truck would be Accessable by anybody.
If you leave it running but take the key with you it will turn off after so long because it doesn’t sense the key anymore.
If you get a truck with remote start that would work. I think the longest it will idle is 15 min.?
Not true.
My in laws borrowed my wife's 2015 Jeep Grand Cherokee with the keyless FOB. They drove off with the car without the key fob. It just so happened that the key was still in the house. We had loaded the car with some things before they left and the car was running. Long story short, they drove 15 minutes away before they realized they didn't have the key and called us.
I don't think the car will automatically shut off without the key fob. Not sure how that would work, to me it seems that would actually be unsafe. If someone did steal or take your car without the key fob inside and it just shut off on the highway that puts a lot of people at risk.
Otherwise I like the keyless FOB. It is safer because you are not fumbling with keys as you approach your vehicle.