I have NMEA 2000 gage senders from Maretron that can be accurately calibrated for any shape tank. The fuel levels can be read on any of the 9 display heads throughout the system. And yes, a LOT of electronics are involved, and both gages have failed once (replaced by Maretron for free).

However I am curious how you guys use a stick. I guess if the fuel tank happens to be particularly accessible, and the filler isn't too long and happens to drop straight into the tank? My tanks are under the quarterberth, so first I would have to remove the bedding and access covers, then unscrew the NPT plug used to purge the sump (aluminum on aluminum, you don't want to do this routinely), dip the stick, clean off the stick so diesel didn't drip everywhere, replace the plug, replace the cover, remake the bed. Probably 1/2 hour.

Thus far, I have not owned a sailboat that would make the use of a stick easy. I do find that the burn rate is pretty dependable at 1.02 gal/hr. And I do have two tanks which makes for a good 1/2 fuel load warning (when the engine quits).

On the OPs problem, just remove the gage sender from the tank and operate it by hand to see what's going on. An ordinary analog gage can be connected to +12 or gnd without damage, my guess is that the wires are connected to the wrong terminals on the sending unit. They look (electrically) like a variable potentiometer, i.e., a resistor with a sliding center tap.