Having driven semi's for 20 years, these are my observations. Old diesels (mechanicals) were hard to start in the winter so once running you kept them running. Most drivers will turn the truck off to fuel. The only time I did not was when my starter went out and so the truck ran until I got back to the yard for repairs. Sometimes in the summer if you shut off a truck the A/C system will vapor lock up and won't get cooling again. It's odd to see someone fuel with their truck running these days. Most will leave them idling when they pull forward to pay.
Sometimes a team will leave the truck running because the co-driver will probably wake up the minute the engine stops.
Be safe and turn your engine off.
