Does your trailer still have the original AC to DC converter (called a 'Univolt', a heavy blue box, probably hums when in use)? Or has it been upgraded to a more modern one?
If upgraded, the person who did the work may not have known to transfer over the "shunt". If this is the case, your tank sensor systems will not work until you have solved the shunt issue.
If you are curious about the shunt, use the Search feature up top in the first dark blue horizontal line.
If it's to be, it's up to me.