would be a big help to know make, model and serial numbers, this sounds to me like the LPG lockoff is not getting power in the "run" position of the igntion switch, but the reason for that depend (like Barfly Olaf says) on "emmission compliant or standard" as to where the power should be coming from. I would check the relays if it's a "CARB" truck

Is it emmission compliant or standard? Could be many things. Check oil pressure light and sensor