The good news, it sips diesel. I had the same boat for years and bought a dowl to stick the depth of the tank. I cut small notches in the dowl to easily determine the level of fuel in the tank. If you open the access panel in the head, you'll find the stainless tank and the fuel shut off valve. Since the tank , fill line and deck plate are aligned, you can measure the distances from the inside the access panel to determine where you put your marks. I notched it in 1/4 full increments. Probably more accurate than the tank guage I have on my current boat.
Hope that helps. Great boat, enjoy!
Mac