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.