Hope I am not out of my league here, but...
...I have a similar problem possibly with my new/used 03 Hunter 306. Rasselas, I see you have a 03 Hunter 36...don't know what type/year Hunter Benny has, but if you have the same three tank indicating system - Acu-gage, by Snake River Electronics - that I have in my 306, there is no float and such troubleshooting for resistance since there is no sending unit in the tank is not appropriate. I think the sensors for the three tanks - waste, water, and diesel are strips on the outside of the tanks. There is a 3-4 page manual in my Hunter book on this system...though I haven't done the troubleshooting to see why, after my first day of my ferry trip from the dealer to my home port, I saw no decrease after about 8 hours of motoring my Yanmar 2GM20F. Of course, I was told the tank was topped off on boat delivery...and I only put less than 5 gallons after the 8 hours. The second day I motored and motor-sailed for about 13 hours - last week...and I haven't topped off yet...shows an actual one tick mark decrease now over full tank...so perhaps it's actually a calibration issue for me. Anyway, I just wanted to point you in the right direction if you have an Acu-gage system...suggest you search in archives on Snake River or Acu-gage...lots of hits and perhaps our betters have already written comments that could help you. Link at snake-river.org also has all the updated information if that is your system. V/r, Dennis