I just mention because diesels require quite a bit of amperage to power the grid heaters and starter. Its not abnormal to have a slight extended crank time, the less amperage the battery is able to produce, the longer the crank time. If your having to start the truck multiple times (assuming actual crank time is 15-25 seconds) then you could possibly have issues. A loss of prime would generally show its ugly head within a few hours of shutdown, not after days.
Has the truck been serviced recently?
Considering you just got the truck it might be a good time to check fuel pressure.
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