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)

Have you checked the LP filter at the throttle body? The filter is a small thin disc located under the LP inlet fitting or under a cover near the LP pressure sensor. The filter can be cleaned and reused.

I will check that out tomorrow. Thanks for the info I will let you know what I find. Also never heard that on the test light, thanks for the heads up.

this "fault", since it sets no codes, means the engine computer thinks it is a "normal" situation, such as when the key switch is turned off, naturally you get no code because of that. (or the LPG fuel running out before the pressure sender).
Here is the thing about "swapping the relays around", the engine computer gets it's power through 2 different (and not next to each other) relays that both have to work, and if either 1 does not work, it is possible the computer sees it as "switched off". In my opinion, it is far better to try a "known good" relay, one socket at a time, or swap out all the relays together.
You may have a bad relay box even, it would not be the first one that ever proved bad.
Someone should also tell you not to use a test light on a 'pulsed signal' like the injectors or fuel trim valves, you can let the smoke out of the black boxes.

No codes at all you can read the hour meter. I swapped the relays around and no change. If you put a test light on the injector fuse it will blink when it tries to start then stop right before it starts. It's a weird one because it runs great if you do get it to start.

Chad, No codes at all? do you see the hour meter reading?. my bet would be the key switch, [the wires to it], if no code at all, but you do see the hour meter.
I would also be wanting to check all the relays by replacing with known good relays.

Did anyone find a fix for this symptom? I have the exact same issue

Check the LPG inline filter by the converter.

these trucks will set codes for almost everything that goes wrong with them, for it not to run but only 5 secs and then quit and not set a code is highly unusual.
best way to see whats going on is to hook up the diagnostic (diagnozer) program and see whats happening.

One fuse to check is on the pigtail plug for the fuel injector,can't be seen to easily as it is in a closed holder,is right at the injector harness under a black cap...

So you DO see an hour meter reading?
Check all the fuses and the key switch itself, then in the blue rely box, there should be a "legend" on the lid, the relay that is for "fuel pump" is a spare in LPG trucks, and can be used to test the other relays, one at a time, swapping them out. if you find a bad relay. my bet (and it is a wild guess) is the 3rd relay.
if it was an ECU related problem, it would have a code, since no code, it is a problem that happens in relation to normal operation, like turning the key off. These trucks sometimes have the key switch get twisted around enough to pull the wires.

No Codes the tank is good seems to be a ECU problem Will run for 5 seconds and you can get full throttle in that time just will not continue running. Good pressures on Fuel and oil. Just wont send spark after the 5 seconds.

this truck should set codes in the dash (where the hour meter reading normally is) if it will not run, if it does not set any codes the key switch or the relays may be at fault. got codes?

What fuel system is the truck....LPG? Gasoline?? diesel?? How many hours are on the truck?
If it is a LP truck check the tank fitting to make sure it is tight,and tank is oriented in the correct position. Does the truck fire back up again after it shuts off? It sounds like a fuel issue to me,a code would be present for most everything else,did someone just change the LP tank and it started acting this way,I have seen where at a tank change the flat gasket in the tank couple gets caught on the Female connector and then you try and screw it onto a new tank and now you have two gaskets in it and it Doesn't screw down tight enough...just something to check...also on top of the NIKKI regulator is a 17 mm hex take it out and put a PSI gauge in it and see what your fuel pressure is!should be about 4.5 PSI.........