Propane gas is heavier than air.
Any leak, no matter how small, will result in your having propane gas in your bilge. So, not too much gas in your bilge will cause a spectacular explosion and fire. Secondly, I have seen propane gas tanks laid on their sides. The relief valves are designed to releas vapor in case of over pressureazation, NOT liquid. If the tank is laying on its side, it will release liquid.