Could one just drain all useable fuel and then add fuel one gallon each side, so two gallons at a time, and mark a wooden stick in one gallon increments? I would think this should work with the airplane sitting in a wings level position. If one gallon is added in each tank and a few moments allowed to self level, it should be very accurate. You could close the fuel valve to stop the tanks from feeding each other and the fuel self leveling, throwing off the readings. Then just fill one tank and take the measurements. But, it might be more accurate, no chance of fuel valve allowing some internal leakage, if each tank was filled one gallon at a time and they self level through the fuel valve.
Am I missing something that would make this idea not work?
You could drain the useable fuel through the belly drain. It would take a while, but it could be done.
I don't understand your definition of "self level." Fuel in the tank is always level. But if you mean the two tanks seeking level with each other, that takes a bit longer. BTW: closing the Fuel Shutoff Valve will not stop the tanks from feeding each other. It only stops fuel from leaving the tanks. They will still vent between each other because they are connected upstream of the valve.
Now, if you get two guys, two ladders, two one-gallon fuel containers, two dipsticks, and they add one gallon to each tank at the same time, they will be level. They could mark each tank and you could compare the results.
Also, if the airplane is completely drained down to unusable fuel thru the belly drain, part of that first gallon will go into the line to the drain. It won't be much, but you will not have a true "one gallon" in the tank.