2000 Chevy Silverado 5.3L Engine P0171/P0174/P0323 Fault - Intake Gasket or Knock Sensor Issue?

Hello, I've searched extensively and haven't found a solution specific to my 2000 Chevrolet Silverado 5.3L engine with 165,000 miles. The check engine light (CEL) is currently on, and the OBD-II diagnostic codes read P0171, P0174, and P0323. While P0323 is commonly associated with a faulty knock sensor, I'm not entirely sure if that's accurate in this case. I recently cleaned the Mass Airflow (MAF) sensor—though it wasn't dirty initially—and believe it’s functioning properly. If anyone knows how to test MAF or other sensors accurately, please share. Several posts suggest a failing intake manifold gasket as a possible cause. However, my truck runs smoothly and has had the CEL on for approximately 1.5 years without any performance issues. Previously, a mechanic advised replacing the oxygen (O2) sensors. When I had the codes scanned at an auto parts store recently, they confirmed that O2 sensors are not the issue. I have limited technical knowledge about engine systems and am seeking advice on what could be causing these codes. Is a failing intake gasket a reasonable possibility? I'm actually selling this truck, so I want to avoid spending significant money on repairs that may not resolve the problem or add value. Interestingly, after clearing the codes at the store, the CEL remained off for about two days—this suggests the issue might be intermittent. Any insights would be greatly appreciated!