Surely your sensors should simply go in the lowest part of the bilge ? Propane, is heavier than air so any significant amount will settle in the bilge.
To my mind the most important part of any gas installation is the line switching. Most people use a Solonoid, switch activated from the cabin, but I hear too many folk complaining about solonoid failure.
When we went gas we ignored the fitting of a solanoid and simply put a pressure gauge on the line. This gauge is visible from the companionway hatch. If it falls below a certain level you know you have a leak. Bottles are turned on and off at the bottles themselves. Yes that is inconvenient but it's a small price to pay as far as we are concerned. Turn bottles on when you start to prepare a meal, turn off at end, safe as safe can be.
“Life is a trick, and you get one chance to learn it.”
― Terry Pratchett, Nation