serial number always helps....
since this unit normally shows set codes in the dash hour meter display, and you don't see them, check the key switch and wiring to the key switch (something that shuts down the truck normally, so that the computer does not 'see' anything out of the usual).
Also you should consider -replacing- all the relays in the relay box (one at a time, if you want, but replace with known good) you may have moved the bad relay to another spot that the truck needs a good relay in to run. it takes at least 3 of the relays all to work (and those 3 may no be right next to each other) for the truck to run.
Just a minor correction that probably does not have anything to do with your problem here and now, assuming this is a LPG powered truck, this unit DOES (or should/did when it left the factory) have 'fuel injectors', just not multi-point fuel injectors, this one has 2 fuel injectors (main or primary, and assistant or helper) mounted in the throttle body.
I would suggest biting the bullet ($) and getting someone who can hook the laptop up to check the sensors, rather than fighting blindly.
this truck NEEDS to see the crankshaft turn before it ever gets spark, and the starter motor turning but the crank sensor not sending a signal to the computer may cause no spark and no codes, since it never thinks the engine is actually turning (the computer does not recognize the engine rpm except from the crank sensor, and does not control the starter cranking.