I am tempted to try and push you into getting Barry to write me for the answer.
;-)
How often is this truck being run out of fuel until it stalls?
Are you using propane that meets the HD5 spec?
Do you know the procedure to clear the codes?
Is the pressure sensor a top mount (flat black sensor mount on the top of the throttle body, held in place with a couple of screws) or a side mount, (pipe thread connection further down on the throttle body)?

On your truck, the "supply voltage" you are seeing is called the "reference" voltage. if you test that to ground (battery negative), you should see about 5 vdc, with the key on. one of the other pins is an almost straight to battery negative (ground) and the one you are not seeing anything on is the one that runs between the sensor and the ECM, and is not really something you can test with a voltmeter.
Your problem sounds [for all the world] to me like the wires are breaking inside the insulation. If and when this happens, it has been my experience that it usually happens right at the connector. I would pull on the wire and see if any of them stretch the insulation, if the wire stretches, then you will have to re-solder those connections, or replace the connector. (there is a kit with that connector, I am pretty sure).

If you happen to Barry, please let him know I tried to give you a hand. I used to fix his forklifts when I lived in Miami. I would love to hear from him again, especially since my wife will be visiting Freeport next May, and we consider him and Birdie to be very good friends.