well depending on what system is on this truck there could be other components in that circuit as edward hinted on.
If we had a serial number so we could get the correct series to make it easier to find the correct wiring diagram that would help.

in general though if this truck has an ECM on it the lpg/ng solenoid shutoff is controlled by it but there are some other common circuits that are tied into that positive side like the starter relay and vsw but those circuits all loop back to the ECM. So if this unit has the ECM control something in the harness or one of those components may be causing the higher resistance blowing that fuse.

If it has the older circuits then it would probably pick up the lockoff solenoid circuit off the ignition relay circuit.

in either case it sounds as if you have something shorted causing the circuit to go high resistance and blowing the fuse.

check the connectors across the main harness at the transmission housing area or just under the dash at the floor plate, where ever the main harness has those large connectors between the engine compartment and the front of the truck, they are known for corrosion problems and i've seen a few harnesses get smashed/pinched because of tech's not paying attention when they are working on them and putting them back together haphazardly.