42-fgc15 sounds like the model number, and 13405 like the serial number.
when this LPG fueled truck runs out of LPG, there is usually about at least 30 seconds where it will run fine at idle speed, but when you give it gas, it will choke and sputter (way too lean fuel mixture as it runs out of LPG).
Sometimes people assume that it ran out of gas when the gas was low as some other problem happened.
Since we are assuming that it is in relationship to changing the gas tank, I would have expected the "real forklift mechanic" to have checked, but you can recheck, to see if the rubber gaskets/o-ring that are supposed to stay in the tank half of the quick disconnect may have come loose and not in the truck half of the LPG quick disconnect valve. if there is any black rubber part in the half of the quick disconnect that connects the hose to the LPG tank, remove them and check for matching missing pieces on the tank 1/2.
ALSO
Since you recently had work done on the motor, and since some of these trucks had an optional low oil pressure shut off, I would be checking to see if this motor had oil and did not have a wire broken off near the side of the motor where the oil pressure sender is mounted, if this unit has a more than 1 wire oil pressure sensor/sender.
Are we clear that currently, this motor starts and runs for 10 (or so) seconds then shuts down? and then can restrat and run for another 10 seconds?