LPG solenoids are spring loaded so that they can easily pass LPG against their normal flow direction. Basically you would drain the line to the tanks with that setup and an Impco converter before it stopped running on gas. The OMVL has a vapour solenoid on it but that only stops part of the flow so I suggest you get a proper underbonnet lock off for the LPG and fit it. These also come with a mandatory filter that stops loose crud from blocking the valve open.
Example of a new one:
LPG Under Bonnet Bowel GAS Lock OFF Solenoid Valve Filter | eBay
You should be able to pick up a second hand one for not very much, there are always a few at every wreckers and some on ebay.