On July 22, 2023, about 1940 central daylight time, an Ercoupe 415-D, N415JC, was substantially damaged when it was involved in an accident near Waupun, Wisconsin. The pilot sustained serious injuries, and the pilot-rated passenger sustained no injuries. The airplane was operated under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. According to the pilot and passenger, they had been en route for about two hours when the engine rpms decreased. The pilot immediately turned the carburetor heat on and cycled the engine throttle control. The engine would sputter and catch but the rpms would decrease again. The engine then lost total power. The passenger stated that the sight gauge on the center (header) fuel tank indicated that the tank was empty. The pilot stated that attempts to restart the engine were not successful and he determined that the engine was likely starved of fuel. The pilot performed a forced landing to a marsh. The airplane came to rest upright with substantial damage to the fuselage, both wings, and the empennage. The flight originated in Conroe, Texas, and was en route to Oshkosh, Wisconsin. The passenger stated that they stopped for fuel in Cedar Rapids, Iowa. He stated that during the fueling the center (header) tank was full with six gallons of fuel, both main fuel tanks were full with nine gallons of fuel each, and each auxiliary tank had two gallons of fuel added. They estimated that they had over four hours of fuel on board when they departed. This was the same fuel load they had used for their entire trip. The pilot stated that after fueling the airplane, the line technician placed the vented fuel cap on the outboard tank backwards. The pilot stated further that it would be about an hour of work to reposition the fuel cap without damaging the airplane, so he chose to address the issue after the airplane flew to and arrived at Oshkosh. According to law enforcement, both main fuel tanks were empty on scene; however, there was a strong smell of fuel in the marsh. The auxiliary fuel tanks were not checked on scene. The passenger and one first responder reported chemical burns on their feet and legs after they stood in the marsh adjacent to the airplane. The left main fuel tank was compromised and the gascolator was impact damaged. A postaccident examination of the engine revealed no mechanical anomalies that would have precluded normal operations. All of the fuel tanks were empty. An examination of the fuel system revealed that the fuel cap on the left auxiliary fuel tank was backwards with the vent facing the trailing edge of the wing.