You did not give a serial number of the truck, and what type of LP system is on it
older trucks had the lock off filter unit and vaporizer
newer ones went to the EPR, and even that was evolved into a DEPR.

if your truck has a wire harness connected to the LP regulator (not a lock off solenoid harness but a multi-wire harness) then it has a EPR or DEPR system on it.

but based on the model you posted this under i found this

under testing and adjusting in service manual based on if this machine has an EPR or DEPR here are the pressure settings for these:
- EPR Primary Pressure: 13.8 to 27.6 kPa (2.0 to 4.0 psi)
- EPR Secondary Pressure (Engine idling with EPR electrical connector connected):
Water column or manometer measurement within ± 19 mm H2O (0.75 in. H2O) of PC Service Tool's "Actual Gaseous Fuel Pressure" value.
- EPR Secondary Pressure (Engine idling with EPR electrical connector disconnected): -25 to -76 mm H2O (-1.0 to -3.0 in. H2O)

- DEPR (PSI) Primary Pressure: 18+/-10 kPa (2.6 +/- 1.45 psi)
- DEPR (PSI) Secondary Pressure: 3.8+/-2.5 kPa (15.25 +/- 10 in. H2O)

If either of these apply to your machine you will need a PC Service tool to check it properly.
If you are not coding out chances are the EPR/DEPR is operating within it's parameters.

There were no other pressure settings listed for any other LP system for the GM 4.3L.

On older systems with the old type regulators they generally had a working pressure output of about 5 to 7 psi.