Holden Rodeo 1999: V6 petrol consumption

My 1999 3.2 V6 Rodeo has been running nicely on LPG for three years, but it is now using about half a tank of petrol per two tanks of gas when running on LPG. I have pulled the petrol pump fuse, but that makes no difference. I notice the injectors are running when it is on gas, but I don't know if this was previously the case. If I put a solenoid valve in the petrol line the injectors would be running dry, which might not be good for them. No one seems to know what to do about this. Where do you suggest I start looking at this problem?