1300 efi as very poor fuel consumption 25/27 mpg if you are lucky, also I have just had my exhaust replaced £250 only available from dealers.
I think the carb and/or timing needs adjusting if you're getting poor fuel economy.
I have a 2004 with a romahome frame on the back. It seems to have a very small fuel tank and only registers 3/4 full when I can't put anymore in it. I agree that it seems to get through fuel very quickly. I am constantly looking to fill it up and have never been able to put more than 25 pounds worth of petrol in it at a time.