We are a retailer/installer of these units and DO NOT install them on LP with any "loose fill" below the ring.
As you pointed out, LP is heavier than air and does "pool" in any nook or cranny below anything. We do LP rings on compacted surfaces like concrete, sheet metal, compacted soil etc.
Just put some gravel in a jar and then pour water into it. It is amazing how much space there actually is for "pooling". Then put in sand or soil and do the same thing. I would recommend going back in and re-doing your fill material.