I added a fuel guage...
...to my '85 H31. It was a secondary project. I had to remove the fuel tank for cleaning. It takes a little doing, but you can get the tank out by way of the galley sink cabinet (at least on the '85).If you have never had the tank cleaned, you may be surprised at the amount of gunk accumulated in there. At any rate, once the tank was out, it was a very straight forward process to cut a hole in top of the plastic and add an aftermarket fuel guage.If you'd rather not remove the fuel tank, then I would support going with the hours approach. It is far easier to add an hour meter to the engine. The 2GMF20 consumes about 1/3 to 1/2 GPH depending on RPM. If you carry a spare jerry can of diesel (5 gal), it's a good backup in case your calculations are wrong.By the way, with an 18 gal tank, if you refill after 30 hours, you're pretty safe. Even with a fuel guage, you're not likely to run below 1/4 of a tank.