I think the main problem would be that fuel tanks have a vent pipe, the LPG tanks are completely sealed. If you were to get a small vapor leek it would/could fill the tray with flamable fumes, as fuel vapor is heaver then air. The other thing with the fuel cells, is you will be filling the tank inside the tray, not externally, there may be an issue with that too.
I could be totally wrong, as i have seen plenty of utes with them.