Drove down to the alps in my lpg-powered 9-5, found this website, stations.gpl.online.fr which is great! Gives you lpg outlets along the route you select, with opening hours, prices and directions. Lpg is 75-78 cents on the motorway, 60 cents a mile off it.
BUT...... where there's cheap lpg there's also cheap petrol, diesel and even ethanol ( for you biopower nuts) so you can plan your stops to eat, rest and fill up, saving about £10 every time.