these did come with nissan engines in them, more than likely since it is a FG15N it would probably be the K21. Would need a serial number to be sure.
But what BLey said is correct, the ecm does control the injector if it is a newer model, the older ones had an LPG Injector driver module, those generally were Tier 1 and early Tier 2, they went away from that in Tier 2 and above eventually incorporating it into the ecm. (if your display has speed and direction indicators it is the newer display which would be late Tier 2 or 3).
You also want to check the Relay Box 2 and look at the EGI relays, they control several circuits power and if they fault out it can cause all sorts of issues. Cat had a recall on those and came out with a newer relay because of it. I'm sure Mitsubitshi did the same since the systems are the same.
As for your E-27 code that is a cam sensor not sending a signal to the ecm for a pre-determined time, an E-28 would be the crank sensor.
I'm surprised it isn't triggering an E-35 code since that LPG injector isn't working properly.
Oh and one other thing, check your connectors on your injector and fuel sensor real good, the wire harnesses in these trucks are bad about breaking wires at or near the connectors where the wires tend to vibrate the most. They changed the core wire in the wiring to a less stranded wire which makes them stiffer and more susceptible to breaking inside.
Cat came out with pigtail connector kits, i'm sure mitsubitshi also did.