There are two fuel tanks in the M577, but they are totally different from what comes in the M113. They are slimmer tanks that run about 3/4 the length of the side sponsons on both sides of the hull. They are connected by crossover tubes as well to keep them balanced. This was originally done as the M577 was supposed to be able to swim.
You can’t really see the fuel tanks here, but they are under the darker green work tables on each side. You can just see the crossover tubes (aluminum) on the sides of the floor.
Underside view.
Crossover tube.
Schematics from TM.
