Seems like a lot of effort to gain a small amount of savings.You're already driving a very economic car I wouldn't complicate it further by adding gas.I'd only save about a grand a year by changing over to LPG.
Maybe start saving for a new Nissan leaf.I might just skip another Prius after my heap is 15. Some of those ev grants are very generous.And the ranges are going up too.If I was to buy one tomorrow the fuel savings would probably pay for the car.