What are the critical diesel pickup maintenance items beyond the manual?

[MarkRJ]

I recently purchased a used diesel pickup truck for towing my travel trailer, and I want to establish a rigorous maintenance routine to ensure its longevity and reliability. I'm familiar with basic oil changes and filter replacements, but diesel engines seem to have more specialized needs, particularly around the fuel system and emissions components. For experienced diesel owners or mechanics, what are the critical maintenance items and intervals you follow that go beyond the standard owner's manual? How do you proactively manage issues like fuel filter water separation, DEF fluid quality, and cleaning the EGR system to prevent costly repairs down the line?