Kev, as indicated IIRC way up this thread, I use
https://www.prix-carburants.gouv.fr/ which is not an app and not in English, but is a French government site that gives comprehensive coverage of French fuel stations. For LPG, just tick that box and then select the département that you're in or travelling through/to from the dropdown menu. It will then show relevant fuel stations on the map. Clicking on an icon will open a popup with prices. Alternatively, click on "Voir la liste des points de vente" and it will bring up a list showing the fuel stations and prices.