Try this
Install a completely full LP tank
Turn the keyswitch on, engine off
With the hood open disconnect the white connector at the bottom of the hood, it should be labelled X14
Reconnect after 2 secs
Turn the key off
Disconnect x14 again
Turn the key on
Reconnect
You sould read a full tank, the code should stop coming
This is a fuel sensor problem. Can be the wrong calibration or technical failure.
Your truck is LPG so, using Pathfinder, you should activate the LPG presure sensor.
Once, Linde had some problems with these sensors, so they issued some Service Information (I don't remember the number) concerning it.
As far as I remember, this required some calibration in Pathfinder window.