Hi i just found worlds largest LPG (autogas, GPL) stations listing site www.LPGstations.com They even have a trip planner future, just i quess you need to sign in firstly.
As well it gives ability to calculate LPG usage. And tags the stations on the rout requered to refill (depending on your avg. mpg i quess). Site is very nice designed and easy to use, have a look.