Po174 Code in 1996 Ford F250 7.5L Diesel - Causes and Fixes After Fuel Tank Switch

HELP... I have a 1996 Ford F25-0 with the 7.5L diesel engine. The truck is regularly used to haul a fifth wheel every 7 weeks, covering about 400 miles round trip and runs very well under normal conditions. However, the check engine light comes on after just a few miles of driving. Last year's smog check revealed that I replaced both the catalytic converter and all three oxygen (O2) sensors. I previously experienced recurring Po174 and Po171 codes. I discovered that when using the rear fuel tank (which operates at low pressure, around 20–22 psi), the code would appear. After installing a new fuel pump, the Po174 code disappeared for one trip — but after restarting the truck after a 10-day idle period, the code returned within just five miles of driving. The engine performs perfectly under load and runs smoothly; however, the persistent check engine light is becoming increasingly frustrating and is now causing my mechanic to suspect deeper issues. We have already performed a smoke test for fuel leaks — results show no leaks. Does anyone else own a 1996 F250 with a 7.5L engine and experience similar Po174 behavior? What are the common causes, especially related to fuel pressure or sensor readings when switching between fuel tanks? Any insights would be greatly appreciated.