I have had this problem with the fuel gauge always showing full since I got our RIB in April. I don't know if it has always been like this or not. The boat is about a year old so should not be a wiring corrosion problem. I fiddled around and got no-where so took it to a local marine specialist in France and they said it was fixed but still showing full after two trips out in her.
I have attached a diagram of a before and after wiring but even the 2nd wiring is not working properly. I have done an ohm check and I think the sender is ok but need to bottom out the correct wiring diagram.
Please advise & many thanks. Nigel