Gremlins are out there!
I've started to have similiar problems. Two weeks ago my 2002 started in the morning, then quit after 5-10 seconds, and wouldn't restart. It was towed to a reputable mechanic who poked around quite a bit, and changed the fuel filter, water sensor and fuel cap. It was fine for a few days, then the same thing happened, this time parked nose down. The first time had been with nose up. I discovered that 3 pumps on the primer (the first 2 had little resistance/pressure) got it going, so it was back to the mechanic. He called TAC, who agreed that it sounds like air in the lines, so the tech installed a clear line between the filter and FICM and ramped up fuel pressure. He started it several times over the next few days, it always started and he found nothing. The second morning I had it back - no start.
I've pressurized with the primer and inspected as closely as I can, from the fuel tank forward. No feel of fuel, nothing visual. This is our only transportation so it needs to be reliable. I've shown my wife how to pump the primer but she's not very happy about doing it if she gets stuck!
Any thoughts would be appreciated.
2002 2wd 2500 EC/LB, Attitude/Juice, Allied 4" Exhaust, 265/75R16 LRE Pirellis, Bilsteins, Timbrens, 34' Automate 5th Wheel RV Trailer