Overall I think the fiat one is reasonably accurate, the issue I have is filling consistently, as it suddenly fills and will spurt back, as a result, I only now fill to the first cut out, which depending on pump flow could be a couple of litres more or less than I have used.
Last October our 130hp Ducato-based Hymer showed 26 mpg for 2000 miles around Italy and Croatia. On the way home it suddenly went to 5.6 mpg. This was on both "A" and "B" journey logs. I have no idea why. I've subsequently replaced the starter battery, so the computer has reset. Any ideas?