Mines running badly at the moment, think I need to remove the EGR, but when running well it will return around 20mpg on gas which is the equivalent of over 40mpg for petrol as LPG is under half the price of petrol.
Disadvantages are mainly the availability of LPG, its not bad near the major road network and cities but stray into the wilds and you're on your own.
Mines done with the gas tank in place of the petrol tank and a 20L auxiliary tank under the drivers seat(under the floor though) this gives very little range on petrol, also the tank stuck out below the chasis rails making it the lowest point of the truck and very vunerable, some 6mm Dural and a body lift got it up behind the chassis and guarded but that wouldn't be everyones cup of tea, if jut for the road not an issue at all though.
Some are done by putting the gas tank across the boot which obviously takes up boot spaces. With the LWB it is possible to have both tanks under the back as there is more space behind the axle.