P0300 and P0301 OBD Codes in 2009 Chevy Silverado: Troubleshooting Guide

Hi there, I own a 2009 Chevrolet Silverado with a 6.0L V6 engine, currently at 120,000 miles. I've recently picked up OBD-II diagnostic codes P0300 (random/multiple cylinder misfire) and P0301 (misfire detected in cylinder 1). The truck runs smoothly at steady highway speeds but exhibits a rough idle when stopping at traffic lights or decelerating. During acceleration, it runs roughly with a noticeable 'putt-putt' backfiring sound—subtle, not loud—but consistent. The check engine light remains steadily on during normal driving and flashes intermittently just after the backfiring occurs. I was advised that poor fuel quality might be the cause. The issue first appeared when the fuel gauge read about half a tank. I recently replaced all spark plugs and ignition wires within the past six months, so I'm confident they're not the root of the problem. I live in Phoenix, Arizona, where temperatures are extremely high right now—could heat exposure be contributing to the issue? Given this, should I replace only cylinder #1's spark plug and wire as a test, or is it better to replace all six plugs and ignition wires at once? Any insights or recommendations would be greatly appreciated. Thanks for your help!