I had a tank tender on my Nordic Tug. We had 2 fuel tanks and a water tank connected to it. There was separate "float type" monitor for the black tank which was located in the keel, so this is what would work (floats). The TT worked great. No problems. For fuel, I also had sight gauges, and I kept track of hours and fuel added. My TT, sight gauges, and hours used always were very close to each other and were very accurate for determining tank level and expected fuel (to add) volume.
The black tank "floats" got "gunked up" once and required to be removed and cleaned, but then worked fine.