Truck will almost turn over when starting, but they dies immediately. Almost like a problem we had before (last year) with dirty throttle body. The fuel filter was changed in 2009 and all other maintenance is up to date. The real question mark is that the truck’s gas cap was found open a week ago (the tank was already almost empty) so not sure if anything was put in the tank or not. No problems till today, 1 week, full tank and 30 miles later.
First thing I would do is check the fuel pressure. You already have suspicions about the fuel supply. The pump will run without the engine running, so check that first. No codes stored?
It does sound like a fuel delivery problem. Try spraying some starter fluid into the intake a see if the engine will run then, if it does then the fuel system has a problem. Check the fuel filter.
Is there a CEL? (Check Engine Light) If so have the code(s) read. Post them back here. They should be in the format [P1234] Some auto-part stores will read them for free.