The pilot reported that, 2 days before the accident flight, the multiengine airplane's fuel tanks were filled (150 gallons). During the taxi to the runway, the right main tire blew. During recovery, the right side of the airplane was placed on a dolly to support the gear so that the airplane could be towed. The pilot reported that, due to the airplane’s fuel system design, when one side of the airplane was raised, all the fuel could be transferred to the opposite tank, which then forced the fuel to be released out of the air vent line. On the day of the accident, the pilot completed his preflight inspection and visually confirmed the fuel quantity by checking both fuel gauges, which were "green"; however, he did not verify the fuel onboard by checking the tanks. About 3 hours into the flight, the rear engine lost power. Before the pilot attempted to restart the rear engine and after he verified the correct engine to feather, the front engine also lost power. When the pilot realized the airplane would be unable to reach the nearest airport, he landed it in a grass marsh with the landing gear retracted. During the landing, the airplane’s wing hit grass and then veered right about 90°, which caused the left wing to dip and impact terrain. The airplane sustained substantial damage to the left aileron and empennage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. During his preflight inspection, the pilot should have verified the fuel quantity in the fuel tanks to ensure there was sufficient fuel onboard for the flight, and his failure to do so led to fuel exhaustion and the subsequent total loss of power in both engines.