Clean out yer AFM with Halfrauds bike chain degreaser, NGK BPR6E spark plugs, decent leads, new air filter, dizzy cap and Rotor Arm. Failing that get your LPG system serviced as it could be fuel starvation. If it's on Petrol try putting a new inline fuel filter in as well.
Forgot to add, my RRC 3.9EFi gets about 26mpg at a steady 65-70 on Petrol, about 20ish on LPG, mixed mode driving about 18-20 on Petrol and about 15ish on LPG.