hello Jazzman,
first, having a serial number would be very helpful.
second, coming here for help was a better idea than throwing parts at the problem.
once we figure out whats on this truck from the S/N we can guide you more effectively, just throwing parts at a problem these days is very expensive and unnecessary.

there are a lot of knowledgeable people here and we can help you fix this :o)

just knowing these engines and systems though a couple things i would say to check right off would be the oil pressure switch, they have been known to fail, if they fail the ignition system is disabled and the engine will not run.
another thing is to verify it is getting fuel, is this lpg fuel?
if so then the lockoff solenoid valve should be operating when the key is turned on, also check the nozzle at the fuel hose and tank and make sure it is screwed completely up on the tank, remove it and make sure the inside of the fuel line nozzle is clear , no other gaskets caught up inside it. When it is screwed up on the tank the gap between the hose side nozzle collar and the tank nozzle should be no bigger than 1/8in or smaller. Sometimes the large round gasket at the end of the male side on the tank will come off getting stuck inside the hose nozzle. When you put a new tank on the truck that keeps it from **** up completely not allowing fuel to go thru.

there may be a couple of other electrical things that may cause it not to get spark but we can deal with that once we know more about this model you have.