Best guess: Fuel related. The cheapest thing to try is change the fuel filter(s). I may be algae in the fuel tank as well. "Good Man" on the irv2 forum, for a similar issue added "a failed fuel cut-off solenoid coul cause the same symptoms.
To check that:
While you are listening at the engine, have someone turn the ignition switch to "ON" and no further.
You should hear a distinctive "click" as the fuel solenoid is energized to pull a plunger allowing fuel to enter the engine. It should also click as the ignition key is returned to the "OFF" position, thus stopping the fuel flow.
If you do not hear this clicking, look for a blown fuse or for some other reason the solenoid is not activating."
and "EnginnerMike said: "- lift pump failed; usually you would notice that at speed while fuel demand is high, but it depends on specifically when & how it fails. True test is pressure test, along w/monitoring of lift pump demand voltage
- injection pump failing; not likely as it would ordinarily be at high fuel demand
- injectors failing; I believe on an 04 you have electronic injectors. These can be read by a shop w/Cat software.
- ECM signal to shut down engine; always (I think) accompanied by a check engine light
- electrical circuit intermittent "open" on fuel delivery someplace; on an electronic injection model there are a lot of possibilities for this. The easy ones are accompanied by gauges shutting down same time or some other indicator that you have lost a main ground or main power feed. I recall reading about a Freightliner chassis problem like this w/an intermittent in the resetting 12V breaker on a cable that ran front to back along the chassis; it was like a 150A breaker and replacing it fixed the intermittent. This would be best addressed under the chassis forum pertaining to your coach, while the more specifically fuel related intermittents would be easier to dope out w/the help of other Cat owners. Since this is a chassis thing, you might try moving to that forum w/the same background & questions."