How many litres of fuel are in the tanks?
Does the hull leak at all and is there ant chance the the tabks could have been under water.
Can you see the sender units and if so what are the flanges /units like there is up to three different metals in these could be a stainless fitting in an Ali tank so if they go under watter they can get quite fizzy.
how old are the tanks and are they Stainless or Aluminium
do you know how old the tubing is and is it the proper gear ISO 7840.
I had these problems twice with internal tanks one was a relatively inexpensive fix and the other was a project.
How much fuel is in the tanks and has it been to sea since you fueled it and if so what are your seperators like
Aologies to all for any typoos