Steve with known fuel in the lines, take the large LPG hose going from regulator/vaporizer to mixer off the mixer. With fuel shut off or disconnected from tank, spin engine over. If fuel comes out then you have a vaporizer that needs replaced. The frost you are concerned of is evap of LPG. Check coolant level also. If unit has vaccum lockoff then check for LPG coming out of vacuum line on lockoff.