I'm selling a slightly used (about 1.5 years) Maretron ultra sonic tank level sender. It attaches to an SAE 5 hole fitting on the tank top, and connects directly to a NMEA 2000 bus. Can be used for fuel (not gas) or water. Although Maretron says it's usable for black and gray water, I have found they don't work well in that application, which is why I'm selling it. But all reports are they they work quite well for fuel and water. One nice feature is that it can be calibrated to report the number of gallons (or liters) in your tank regardless of the tank shape. Please note this is the TLM200 which can be used for tanks as deep as 108". It's cousin the TLM100 is only good for 40".
Here is a link to the product page including installation and other manuals
Maretron | TLM200
Typical online pricing is about $600, so I'll offer it for $300.