P0300 Fault Diagnosis on 2001-2004 Chevy 8.1L V8 Engine in Motorhome Application

I recently installed a factory test unit of the 2001-2004 Chevrolet 8.1L V8 engine into a custom-built 6x6 motorhome. The standalone fuel injection harness and ECU were purchased from Fuel Injection Specialties of San Antonio, but I have encountered significant issues with their product quality and support — including multiple wiring errors in the sensor connections. Specifically, the camshaft position sensor plug did not reach its mounting point and was incorrectly wired; the crankshaft position sensor connector and coolant temperature sensor were also improperly connected. After being sent replacement connectors, I was instructed to splice them in — resulting in 11 total splices (including cam, crank sensors, and knock sensors). Given these wiring errors, could a faulty splice be directly causing the P0300 random misfire code? Additional observations: The engine fails to fire any single cylinder when ambient air temperature drops below 70°F. Starting fluid is required to initiate combustion. Once running, the engine idles and runs roughly despite having spark plugs with good color (though possibly indicating a lean mixture). I have already tested various injectors by unplugging them during operation — no noticeable improvement. I plan to check fuel pressure today to rule out delivery issues. All sensors and components are original GM parts, including the fuel pump, which was installed within 6 inches of the fuel source with a filter positioned between the pump and the fuel tank.