it is possible that the vaporizer has some sort of internal failure
this will create this issue with no codes being displayed

generally if there are any issues with relays or fuel delivery at the throttle body, fuel sensor etc it will trigger some sort of code.
clean out the vaporizer by removing the drain plug on the bottom of it, also make sure the coolant level in the cooling system is full, low coolant can also keep the vaporizer from functioning properly.

also check the fuel lockoff, it can do this same thing , either a no start or a start and shutdown shortly afterwards with no codes being displayed.

found this info from some service bulletins on this symptom.

as for the relays in the relay box
like edward says, there are 2 relays that control the EGI circuit and they are not side by side, the order of relays is as follows and should also have a sticker in the lid of the relay box stating this too.
-EGI relay2 (ecm)
-MainC relay (lp fuel lockoff)
-EGI relay1 (ignition coil)
-ETC motor relay
-Gas fuel pump relay (not used on LP trucks but is installed usualy)