On a vehicle like a P38 there's not much chance of the LPG install preventing other ECU's going to sleep. That can potentially occur on a minority of vehicles if the LPG ECU is wired to canbus to read the vehicle's fuel trims (by way of keeping talking to ECUs when ignition is off thus keeping ECU's awake) but that wouldn't happen on a P38 even if the LPG ECU was connected to the OBD socket. The only way the LPG ECU would be likely to draw current with the ignition off is if it's ignition on sensing wire is connected to something that's constantly powered up.