there are two things to look at
first is the fuel lock solenoid, that opens flow from the tank into the filter and/or evaporator(converter).
Now some LP systems have a vacuum hose running to a filter assy that is hooked inline with the converter, in this case there is probably no lock-off solenoid because the filter is actually the fuel lock valve and opens when the engine pulls a vacuum on it.

secondly there is a diaphragm in the mixer (carb) that also has a shut off (bullet valve) on it.

If you hear it rushing into the mixer (carb) then the mixer diaphragm is probably bad and should be replaced.