Isuzu Repair: 1997 Holden (Isuzu) Rodeo 2.6, holden rodeo, isuzu rodeo

Question
I recently purchased a 1997 2.6 litre Holden Rodeo.  It has been converted to LPG.  I recently serviced it with a new  filters and fresh oil.

It is returning 17.5 litres per 100km's. I know LPG is not as efficient as petrol but it can't be this inefficient.  I think O2 sensor but I cannpt locate it.  

What do you think?

Answer
Well if it has been converted to Lpg then I would be giving you a wild guess about what it might be Rel just to be honest with you. It would all just be speculation on my part, However, I am willing to help you figure it out with a little TLC if you prefer.