I had a 4.6 litre Land Rover on LPG, and at 13 mpg I thought it was pretty awful, but, it usually delivered the same economy as all the other vehicles that used to tow our club boat. Sometimes it would even beat the heavier footed individuals!
You can now get overlays to download into GPS units for the stations. What will kill it will be any governement interference. I'm planning a second conversion based on the 4.6 engine with more efficient carbs and a better cam profile.
If the duty does change then it will be straight to diesel for me, possibly with a sensible LPG kit on that to improve burn.
New boat is here, very happy!